[2]

(3 575

Genus Tetraclita

in T. versicolor.  On along the occludent edge, headmargin the inflected end extremity of each alternate growth-ridge is generally much thickened,─ a set of teeth being thus formed, by which the two valves are locked together, but In the T. porosa, this character is variable, for instead of sometimes every alternate ridge, & sometimes only every two or three ridges, distant separated from each other by several growth-ridges, are thus developed into teeth. The articular ridge is either moderately prominent or extremely prominent as in T. versicolor cœrulescens or [illeg];but the lower edge in no case depends as a freely free, hinge-like style as sometimes in Balanus. The adductor ridge is generally well developed & quitedistinct from the articular ridge: in T. purpurascens it is very blunt: in T. serata it joins is united to the articular ridge half way up it, thus forming a deep tubular cavity above the crests from the lateral depressor muscle running up to the apex of the valve: in T. versicolor cœrulescens, the adductor ridge is very short & is united to or almost continuous with the bottom lower end of the articular ridge, a small sub-cylindrical tubular cavity being thus formed. Small crests exist for the attachment of the rostral & lateral depressor muscles in all most of the species, excepting T. purpurascens & costata, in which, however, more especially in the former there are, instead of crests, sma minute

[2v]

[George Darwin:] Manual exercise

Supposed to be drawing

[child's annotated drawing of a gun]