PLATE 92. 



Page 

 Fig. I a - /I. DiSCINA GRANDIS. 406 



1 a. The dorsal valvo, from a cast taken from the mould of the original in sandstone. 



1 4. Tlie ventral valve from the inside, the shell having been mostly removed l)y weathering. 



1 c. Profile of the dorsal valve. 



1 d. Tlio ventral valve, from a cast retaining the two valves in connexion. Tlie foramen, as 

 seen in this specimen, e.xtends more nearly to the margin than is shown in fig. 1 b, whicli 

 is seen from the inside. 



Fig. 2 & 3. Steophodonta maoniventua. 411 



2 a. Tlie cast of the interior of a ventral valve, where the muscular impression is very strongly 

 marked, and occupies a large part of the area of the valve. The central upper portion, 

 marked by the adductor muscles, is, in this and some other specimens, well defined and 

 separated from the other parts. 



2 6. A similarcast, where the cardinal angles are more extended and the muscular impressions 

 less strongly defined. Both this and the preceding specimen show the imprint of the strong- 

 ly striated cardinal area; the first one having a flattened space without striae in tlie place 

 of the foramen, while this feature is less distinctly seen in tlie latter. 



2 c. A fragment of a similar specimen, showing a process extending into the cavity towards 

 tlie beak of the valve. A mould made from the casts of this species shows the foramen to 

 be entirely closed, with a thickened process below and a cavity extending beneath it to- 

 wards the beak; as if there may have been, at some period of growth, a perforation of 

 the apex. 



3. The exterior of the ventral vaive, showing the strongly striated surface. 



Fig. 4. Steophodonta vasculaeia. 412 



4. A cast of the ventral valve. ( The muscular and vascular markings are not correctly re- 

 presented in the figure.) 



