232 CRETACEOUS PELECYPODA 



cither side into which the two cardinal teeth of the upper valve fit ; one or more 

 rido-es are usually seen in the interior of the valves extending from the umhones 

 to near the poriphci-y ; muscular impressions on more or less projecting rugose 

 ridgos, generally at the base connected with the hinge-teeth ; pallial line simple ; 

 ligament situated in a narrow groove extending from the umbo of the lower valve 

 to the hinge ; in the upper valve this groove is less distinct and appears to he some- 

 times obsolete. Woodward states that the uneven ridges on either side of the liga- 

 mental groove support an internal cartilage. This may be the case, and certainly 

 when looking on a cast, as that of C. qiiadrijxirl/ta, it appears to be very proljable, 

 but if we judge from analogy, these rough or ribbed parts appear to be the same 

 Avhich in SpJucnd'ttes and EadloUtes are said to support the retractile muscles of 

 the valves. Should, however. Woodward's explanation prove to be correct, this and 

 the few following genera will have probably to be associated with the Chamostreid^, 

 as already noticed. But I am not sure whether all these fossil genera are really 

 sufficiently distinct. Good materials may necessitate considerable reductions in 

 their number. 



The hinge of Capvot'nia is almost quite the same as that of Monoplenra, with 

 the only exception that the muscular impressions are on strongly raised prominences 

 and tlie hinge-teeth themselves much stronger in the former genus. Pictet and 

 Campiche in their last reviews of this family have restricted the name Caprotina 

 to a few species only, namely, such forms as have the lower valve slightly spiral, 

 with a broad place of attachment, the upper valve somewhat convex and with 

 a marginal pointed and svib-S[)iral umbo, and no sulci on the external surface 

 of the shell. As compared with Caprbia, the present genus only ditfers from it 

 by the more equal form of the two cardinal teeth in the free valve. 



9. DiplkUa, Math., 1842, (Cat. Meth., &c., p. 111). Shell inequivalve, solid, 

 concentrically lamellated and finely radiately striated ; attached valve conical, elon- 

 gated with a single longitudinal furrow on one side ; free valve slightly convex, with 

 an eccentric, sub-marginal, pointed umbo and a groove from it running to the 

 margin and corresponding with that of the other valve; hinge not known ; type, 

 D. tmisulcafa, Math., from upper cretaceous beds at Martiques. Pictet and Cam- 

 piche (Pal. Suisse, 5""' ser., 4™' part., p. 34,) suggest that until there is something 

 more definite known of Matheron's type species of Diplidia, the genus should 

 be retained as characterized by its author. D'Orbigny placed it in Caprotina, 

 but had not seen a specimen of the species. There can be no doubt that the peculiar 

 conical and obtusely pointed lower valve offers some distinction from Caprotina, and 

 entirely resembles that of a SphcBruUtes. But from this the genus again differs by the 

 form of the free valve. The want of any external dorsal sulci separate it from JIoiio- 

 pleura, and thus it seems best to retain provisionally the genus as originally defined. 



The same applies to the two next genera, both of wdiicli are as yet little known, 

 and have been referred by Woodward, on the suggestion of Sharpe, to a single 

 genus ; but it appears preferable to keep them provisionally distinct. Both have 

 most likely to be referred to the Cbamostbeidj!, especially if the so-called ligamental 



