ONITHOCHITON. 245 



outside, that of the first valve with 8 slits, median valves 1 slit ; pos- 

 terior valve depressed, triangular, with posterior terminal and mar- 

 ginal mucro, the insertion-plate reduced to a low, smooth and narrow 

 callus. Girdle leathery, rendered velvety by very minute chaffy 

 hairs. Type 0. undulatus Quoy. 



This genus resembles Enoploeliiton and Liolophura in the char- 

 acters of the tail valve. It differs from the former in the unsculpt- 

 ured, porcellanous interior, much more delicate structure of the tail- 

 valve callus and different type of girdle. Onithoehiton differs from 

 Liolophura in the texture both inside and out, the very different 

 arrangement of the eyes, more delicate tail-valve callus, and in 

 the characters of the girdle. 



The species are distributed from the Cape of Good Hope to New 

 Zealand. 



Carpenter emended the name to " Ornithochiton " believing it to 

 be derived from ornis, ornithos, in allusion to the resemblance of 

 single valves to the conventional representation of a bird on the 

 wing ; but if any Greek root was in Gray's mind it was probably 

 Onychochiton rather than Ornithochiton. 



Key to species of Onithoehiton. 



a. Lateral areas and anterior valve smooth or radially sculptured ; 

 central areas smooth. 

 b. Lateral areas smooth or with 4 or 5 low subgranose riblets, 



undulatus. 

 bb. Lateral areas closely sculptured with close, subgranose striae, 



semisculptus. 



aa. Lateral areas having concentric sculpture ; central areas or 

 pleura with for wardly-con verging sulci. 



b. Length of posterior valve from sinus to mucro distinctly less 

 than half the valve's breadth, lyellii, quercinus, rugulosus, 



[amicorum. 



bb. Length of posterior valve from sinus to mucro half the valve's 

 breadth. 



c. Valves having a smooth dorsal triangle, maillardi. 



cc. Valves having zigzag sculpture over the ridge, literatus. 



O. UNDULATUS Quoy & Gaimard. PI. 55, figs. 14, 15, 16. 



Shell oblong, moderately elevated, the dorsal angle rounded, side- 

 slopes nearly straight. Surface polished; color either (1) olive-buff 



