232 CRETACEOUS PELECYPODA 



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

 ridges are usually seen in the interior of the valves extending from the umhones 

 to near the periphery ; muscular impressions on more or less projecting rugose 

 ridges, 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 be some- 

 tiDies 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. quadripartita, it appears to be very probable, 

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

 Avhich in SplK^rulites and RadioUtes 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 Chamostr:eii)^^, 

 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 Caprotina is almost quite the same as that of Monopleava, with 

 the only exception that the muLScular impressions are on strongly raised prommences 

 and the 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 sub-spiral umbo, and no sulci on the external surface 

 of the shell. As compared with Gaprina, the present genus only differs from it 

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



9. Diplidia, 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 runnino- to the 

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

 D. unis'ulcala, 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 DipUdla, the genus should 

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

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

 conical and obtusely pointed lower valve off'ers some distinction from Gaprotlna, and 

 entirely resembles that of a Sph(jeralites. But from this the genus again difi'ers by the 

 form of the free valve. The want of any external dorsal sulci separate it from Mo7io^ 

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



Tlie same applies to the two next genera, both of which 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 Gramostreibj^., especially if the so-called liganiental 



