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Mr. Waterhouse resumed the exhibition of the small Rodents, belonging to the collection presented by Mr. Darwin to the Society. Among them were three species allied to the genus Mus, but offering some slight modification, not only in the external form, but in the structure of the teeth. They have the fur soft and silky; the head large, and the fore legs very small and delicate; the tarsus moderately long and bare beneath; in the number and proportion of the toes they agree with the true rats; the tail is moderately long, and more thickly clothed with hair than in the typical rats. The ears are large, and clothed with hair. Like the true rats, they have twelve rooted molars; the folds of enamel, however, penetrate more deeply into the body of each tooth, and enter in such a way that the crowns of the teeth are divided into transverse and some what lozenge-shaped lobes, or in some instances into lobes of a triangular form. In the front molar of the upper jaw the enamel enters the body of the tooth twice, both on the outer and inner sides; and in the second and posterior molars, both of the upper and under jaws, the enamel penetrates but once externally and in-

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ternally in each. In the front molar of the lower jaw the enamel enters the body of the tooth three times internally, and twice externally.

As the above-mentioned characters, in Mr. Waterhouse's opinion, evidently indicated an aberrant form of the Muridæ, he suggested the propriety of constituting a subgenus under the name of Phyllotis* for the reception of the species.

They were characterized as follows :—

MUS (PHYLLOTIS) DARWINII. M. suprà pilis cinnamomeis et nigrescentibus intermixtis; ante oculos cincrascentibus; genis, lateribus corporis, et caudâ prope basin, fulvo-cinnamomeis; partibus inferioribus pedibusque albis; auribus permagnis, ferè nudis; caudâ caput corpusque ferè æquante, suprà fusco-nigricante, subtùs albâ.

	unc.	lin.
Longitudo ab apice rostri usque ad caudæ basin	6	0
——— caudæ ……………	4	9
——— ab apice rostri ad marginem oculi.	0	8¼
——— ab apice rostri ad basin auris ….	1	4½
——— tarsi digitorumque …………	1	1½
——— auris ……………	0	11¾
Auris latitudo …………………	0	11¾

Hab. Coquimbo.

MUS (PHYLLOTIS) XANTHOPYGUS. M. suprà pallidè brunneus flavo-lavatus, ad latera flavescens, subtùs albus; capite griscescente; natibus flavis; pedibus albis; auribus majusculis pilis albis et flavis intermixtis obsitis; caudâ longitudinem corporis ferè æquante, suprà nigricante; subtùs albâ; vellere longo et molli; pilis corporis omnibus ad basin plumbeis; mystacibus perlongis albescentibus, ad basin nigris.

	unc.	lin.
Longitudo ab apice rostri ad caudæ basin ..	5	3
——— caudæ ……………	3	10
——— ab apice rostri ad marginem oculi.	0	6¾
——— ab apice rostri ad basin auris ….	1	3
——— tarsi digitorumque …………	1	1
——— auris ………………	0	7
Auris latitudo ………………	0	6½

Hab. Santa Cruz.

MUS (PHYLLOTIS) GRISEO-FLAVUS. M. suprà griseus flavo-lavatus, ad latera flavus, subtus albus; pedibus albis; auribus magnis et ferè nudis; caudâ caput corpusque ferè æquante, suprà fusco-nigricante, subtùs albâ; vellere longo, molli; pilis ad bases plumbeis.

* Phyllotis, from Φυλλον, a leaf, and Ους, ωτος, an ear.

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	unc.	lin.
Longitudo ab apice rostri usque ad caudæ basin	6	8
——— caudæ ……………	5	6
——— ab apice rostri ad marginem oculi.	0	8
——— ab apice rostri ad basin auris ….	1	4½
——— tarsi digitorumque ………	1	2½
——— auris ………………	0	8½
Latitudo auris ………………	0	8½

Hab. Rio Negro.

This species may be readily distinguished from M. xanthopygus by the greater proportionate length of its tail.

Two species of small Rodents were next characterized as constituting examples of a new genus, for which Mr. Waterhouse proposed the name of

REITHRODON.*

"Dentes primores 2/2; inferioribus acutis, gracilibus, et anticè lævibus; superioribus gracilibus, anticè longitudinalitèr sulcatis.

Molares utrinque 3/3 radicati; primo maximo, ultimo minimo: primo superiore plicas vitreas duas externè et internè alternatìm exhibente; secundo, et tertio, plicas duas externè, internè unam: primo inferiore plicas vitreas tres externè, duas internè; secundo, plicas duas externè, unam internè; tertio unam externè et internè, exhibentibus.

Artus inæquales: antipedes 4-dactyli, cum pollice exiguo unguiculato: pedes postici 5-dactyli, digitis externis et internis brevissimis.

Ungues parvuli et debiles. Tarsi subtùs pilosi.

Cauda mediocris, pilis brevis adpressis instructa.

Caput magnum, fronte coo: oculis magnis: auribus mediocribus.

"In the present ge, the incisors, compared with those of the true rats, are rather smaller in proportion, and those of the upper jaw also differ in having a longitudinal groove, a character which exists in Euryotis (Brants), Gerbillus, Otomys (Smith), Dendromys, and some other genera, but not combined with molars similar in structure to those above described, nor yet with similar external characters. In other respects the incisors resemble those of the genus Mus; that is to say, those of the lower jaw are long, slender, and pointed, and those of the upper are deep from front to back, and somewhat flattened at the sides and in front. The molars gradually decrease in size from the front to the last posterior tooth. The folds of enamel penetrate deeply into the crowns of these teeth, so that those from one side are in contact with those of the other; these folds of enamel are each nearly opposed to the salient angles of the opposite side.

"In the two species of this genus with which I am acquainted the fur is long, very soft, and consists of hairs of two lengths. The

* Ρειθρος, a channel; Οοον, a tooth.

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arched form of the head and the large eyes produce in these animals a slight resemblance to young rabbits; their affinity, however, is with the Muridæ."

REITHRODON TYPICUS. Reithr. vellere suprà pilis flavescenti-fuscis et nigrescentibus intermixtis composito; regione circa oculos, genis lateribusque corporis auratis, pilis pallidè fuscis intermixtis; partibus inferioribus auratis; rhinario ad latera flavescentialbo; auribus magnis, intùs pilis flavis, extù flavis et fuscis, indutis; caudâ suprà pallidè fuscâ, subtùs sordidè albâ pedibus albis.

	unc.	lin.
Longitudo ab apice rostri usque ad caudæ basin	6	0
——— ab apice rostri ad marginem oculi.	0	8¼
——— ab apice rostri ad basin auris ….	1	4½
——— tarsi digitorumque …………	1	2½
——— auris ……………	0	8½
Latitudo auris ……………	0	8½

Hab. Maldonado.

REITHRODON CUNICULOĪDES. Reithr. suprà griseus, flavo-lavatus, pilis nigris intermixtis; abdomine gulâque pallidè flavis; natibus albis; pedibus albis; auribus mediocribus, intùs pilis flavis, extùs pilis pallidè obsitis, maculâ nigrescente ad marginem anteriorem postâ; pone aures, notâ magnâ albescenti-flavâ; caudâ corpore breviore, suprà pallidè fuscâ, subtùs albâ.

	unc.	lin.
Longitudo ab apice rostri usque ad caudæ basin	6	5
——— caudæ ……………	3	3½
——— ab apice rostri ad marginem oculi.	0	9 1/3
——— ab apice rostri ad basin auris ….	1	4
——— tarsi digitorumque ………	1	4½
——— auris ……………	0	7

Hab. Santa Cruz.

In conclusion, two other new Rodents were characterized under the generic name of

ABROCOMA.*

Dentes primores 2/2 acuti, eradicati, anticè læves: molares utrinque 4/4 subæquales, illis maxillæ superioris in areas duas transversales ob plicas vitreas acutè indentatas divisis; plicis utriusque lateris vix æquè profundis; illis mandibulæ inferioris in tres partes divisis, plicis vitreis bis internè, semel externè indentatis, areâ primâ sagittæ cuspidem fingente, cæteris acutè triangularibus.

Artus subæquales.

Antipedes 4-dactyli, externo brevissimo, intermediis longissimis et ferè æqualibus.

* 'Αβρος soft; Κομη, hair.

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Pedes postici 5-dactyli; digito interno brevissimo. Ungues breves et debiles, illo digiti secundi lato et lamellari; omnibus setis rigidis obtectis.

Caput mediocre, auribus magnis, membranaceis; oculis mediocribus.

Cauda breviuscula.

Vellus perlongum, et molle.

"The genus Abrocoma is evidently allied on the one hand to Octodon, Ctenomys, and Pœphagomys, and it appears to me almost as evidently allied on the other hand, to the Chinchillidæ. The dentition, however, differs considerably from either of the above-mentioned genera, or, from either of those of the family Chinchillidæ, and in fact indicates a new generic form*. From Ctenomys and Pœphagomys the present genus is readily distinguished, by the comparatively large size of the ears, the small delicate claws, and smaller size of the incisors; and from Octodon by the uniform length of the hairs on the tail.

"In the structure of the feet the genus Abrocoma approaches very nearly to Octodon, not only in the form, but in having the soles both of the fore and hind feet (which are devoid of hair) covered with minute round fleshy tubercles. In Octodon, however, the toes have on their under side transverse incisions as observed in the Muridæ, a character, however, not found in Abrocoma; here the under side of the toes is, like the sole of the foot, covered with tubercles.

"The extreme softness of the fur of the animals about to be described, suggested for them the generic name of Abrocoma. The fur consists of hairs of two lengths, and the longer hairs are so extremely slender that they might almost be compared to the web of the spider. The specific names applied are those of the distinguished naturalists who first made us acquainted with the two genera Octodon and Pœphagomys, these being very nearly allied to Abrocoma."

ABROCOMA BENNETTII. A. corpore suprà griseo, ad latera pallidiore et pallidè cervino lavato, subtùs albescenti-cervino; gulâ albescenti-griseâ; pedibus sordidè albis: auribus amplis, ad marginem posticum rectis, fere nudis, attamen extùs ad bases vellere, sicùt in corpore, obsitis: caudâ corpore breviore, ad basin crassiusculâ, pilis brevibus incumbentibus vestitâ.

	unc.	lin.
Longitudo ab apice rostri usque ad caudæ basin	9	9
——— caudæ ……………	5	0
——— ab apice rostri ad marginem oculi.	0	11½
——— ab apice rostri ad basin auris ….	1	11
——— tarsi digitorumque …………	1	4
——— auris ……………	0	10
Latitudo auris ………………	1	0½

Hab. Chili.

* "I may here mention that the folds of enamel in the dentition of the lower jaw very much resemble those in the teeth of the genus Arvicola."

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ABROCOMA CUVIERI. Ab. suprà grisea, levitèr ochraceo lavata; abdomine gulâque albescenti-griseis; pedibus sordidè albis; auribus amplis, ad marginem posticum distinctè emarginatis, ferè nudis attamen extùs ad bases vellere, sicùt in corpore, obsitis: caudâ corpore multò breviore, et nigrescente.

	unc.	lin.
Longitudo ab apice rostri usque ad caudæ basin	6	6
——— caudæ ……………	2	10
——— ab apice rostri ad marginem oculi.	0	6¾
——— ab apice rostri ad basin auris ….	1	4
——— auris digitorumque ………	1	1
——— auris …………	0	7
Latitudo auris ……………	0	7½

Hab. Valparaiso.