654 



MOLLUSCOIDEA. 



to some foreign object. The dorsal valve is conical or limpet- 

 shaped, and the shell in both valves is tubulated. In the interior 

 of each valve are four principal muscular impressions, formed by 

 the adductors, and in the interior of the ventral valve, centrally, is 

 placed a triangular protuberance, which serves to support the bases 

 of the fleshy arms. The family ranges from the Ordovician to the 

 present day. 



The only well-established genus in this family is Crania itself, 

 which commenced to exist in the Ordovician period, and which is 

 represented at the present day by several living species. The shell 

 in Crania may be smooth, or striated with radiating ribs, sometimes 

 with spines or foliaceous expansions, while the internal margins of 

 the valves are expanded and often granulated. The ventral valve 

 is fixed by its lower surface to foreign bodies, while the free dorsal 

 valve is more or less conical (fig. 501). 



In the sub-genus Pseudocrania (Ordovician to Devonian), the shell is 

 only slightly inequivalve and is free, while the inner margins of the 

 valves are smooth. In this family may perhaps be placed the singular 



Fig. 502. — Trimerella 

 acuminata — cast. Silu- 

 rian. (After Davidson and 

 King.) 



Fig. 503. — Trimerella 

 grandis — cast. Silurian. 

 (After Davidson and 

 King. 



Fig. 504. — Monomerella 

 prisca — cast. Silurian. (After 

 Davidson and King.) 



Ordovician genus SchizOcrania, in which the shell resembles Crania in 

 general characters, and is fixed by the flattened ventral valve, while the 

 dorsal valve is conical and is radiately striated. The ventral valve, how- 

 ever, exhibits a triangular notch on its posterior side, extending nearly 

 to the centre, and there are six muscular impressions in the dorsal 

 valve. 



Family 5. Trimerellid^e. — This family is related to that of the 

 Obolidce, and comprises forms in which the shell is thick and cal- 



