ORTHIDES OP THE HAMILTON GROUP. 47 



J?. Species of the t3'pe of Orlhis hybrida and Orthis obiata. 

 Orthis vanuxemi, Orthis leucosia, Ohthis penelope, Orthis cyclas. 



Oitliis yaiiaxemi. 



PLATE VL 

 Orthis tanuxemi : Hail, Tenth Report on the State Cabinet, 1857, p. 136. 



Shell subcircular or transversely suboval, compressed : hinge-line very- 

 short ; margins of the valves crenulated within from the external strife ; 

 interior minutely punctate. Dorsal valve convex : beak scarcely dis- 

 tinct from the cardinal border, not incurved ; cardinal process promi- 

 nent ; area flat or slightly inclined to the ventral area, and about two- 

 thirds as wide. Ventral valve nearly flat or a little concave towards the 

 front, moderately convex in the umbonal region : beak small, extending 

 little beyond the opposite beak, arched, and rarely incurved over the 

 area ; area very small, less than half the greatest breadth of the shell, 

 arcuate ; foramen comparatively large, triangular, and partly filled by 

 the cardinal process of the other valve : teeth prominent. 

 Surface marked by fine, closely arranged, radiating tubular striae, which 

 are perforate at intervals, increasing both by implantation and bi- 

 furcation, and are crossed by very fine indistinct concentric striae, and, 

 at greater intervals, by more distinct, concentric, imbricating lines of 

 growth : entire surface granulate or punctate, under a magnifier. Striaa 

 from twelve to sixteen in the space of two lines near the beak, and 

 from seven to nine in the same interval near the margin. 

 The interior of the dorsal valve shows a strong cardinal process, which 

 is continued in a prominent rounded median ridge for half the length of 

 the shell, where it sometimes divides, or gradually becomes obsolete : 

 there are sometimes visible low transverse ridges, which divide the 

 muscular impression. The crural processes are prominent, and sustained 

 below by strong oblique ridges. In the interior of young specimens, the 

 marks of the external striae are visible nearly or quite to the muscular 

 impression ; while in older specimens these marks extend little beyond 

 the margin. 



