458 



MOLLUSCA. 



The genus, or sub-genus, Strophalosia (fig. 311, B) ranges 

 from the Devonian to the Permian, and is distinguished 

 from Producta chiefly in the fact that the valves are not 

 edentulous, but are articulated by teeth and sockets, while 

 each valve has a distinct hinge-area, and the ventral valve 

 has a foramen covered with a deltidium. Aulosteges, again 

 (fig. 311, A), from the Permian, has no teeth or dental sockets 

 in this respect agreeing with Producta but the ventral 

 valve has a wide hinge-area, pierced by a foramen, which is 

 covered by a convex pseudo-deltidium. Lastly, in the De- 

 vonian Productella (fig. 311, c) the valves are articulated by 



Fig. 311. A, Aulosteges Wangeriheimii Permian, showing the hinge-area and deltidium ; 

 B, StropJialosia Goldfussi, viewed dorsally Permian ; c, Productella onusta Devonian in- 

 terior of the dorsal valve, showing the cardinal process (c), the muscular scars (m), and the 

 reniform vascular impressions (v). (After Davidson and Hall.) 



teeth and sockets, and a hinge-area is present in both valves, 

 but the latter is narrow and linear, and the ventral valve is 

 extremely convex and gibbous. All the three groups just 

 mentioned are probably to be regarded as really nothing more 

 than sub-genera of Producta itself. 



FAM. VIII. CRANLAD^E. Animal fixed to submarine ob- 

 jects by the substance of the ventral valve. Arms fleshy 

 and spirally coiled. No hinge or articulating processes ; 

 upper or dorsal valve limpet-shaped (fig. 312). The family 

 includes the single genus Crania, which commenced in the 

 Silurian period, and has continued to exist without inter- 

 ruption to the present day. The shell may be smooth, or 

 striated with radiating ribs, sometimes with spines or foli- 

 aceous expansions. The ventral valve is the shallowest, and 

 the shell is usually attached by a portion of its substance. 



