PLATE 92. 
Page 
Fig. 1 a - ri . Discina grandis. 406 
1 a. Tlie dorsal valve, from a cast taken from the mould of the original in sandstone. 
1 b. The ventral valve from the inside, the shell having been mostly removed by weathering. 
1 c. Profile of the dorsal valve. 
1 d. The ventral valve, from a cast retaining the two valves in connexion. The foramen, as 
seen in this specimen, extends more nearly to the margin than is shown in tig. 1 b, which 
is seen from the inside. 
Fig. 2 & 3. Stropiiodonta magniventra. 411 
2 a. The 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 b. A similar cast, 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 strise in the place 
of the foramen, while this feature is less distinctly seen in the latter. 
2 c. A fragment of a similar specimen, showing a process extending into the cavity towards 
the 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 valve, showing the strongly striated surface. 
Fig. 4. Strophodonta vascularia. 412 
4. A cast of the ventral valve. (The muscular and vascular markings are not correctly re¬ 
presented in the figure.) 
