22 CRETACEOUS PELECYPODA 



orientalis, Gmel., is the type of this genus which certainly is closely allied to 

 Barnea and has, I believe, only sub-generic value. 



15. Fholas,^ Linne, 1757, (H. and A. Adams, Gen. ii, p. 325). Shell elon- 

 gated, one umbonal accessory plate, extending forward, and one small post-umbonal ; 

 hinge plate reflexed over the beaks and closely applied. 



Klein used the name Fholas correctly, but his figure of the type species is not 

 clear. 



15a. Tryon separates the species with a wide emarginated anterior hiatus (like 

 Ph, crucifera, Sow.,) into a distinct sub-genus, which he calls Cyrtopleura (1862). 



16. Dactilina, Gray, 184i7, (H. and A. Adams, Gen. ii, p. 825,) differs from 

 Pholas in having two accessory, umbonal valves placed side by side ; the hinge plate 

 is cellular beneath, reflexed over the beaks. 



The typical species of Dactilina are those with a short emarginated hiatus in 

 front, like Z>. datylus, 



IQa. Another group with entire margins of the valves along the anterior 

 hiatus, with the nuclei of the dorsal valves placed anteriorly, like D. Campechensis of 

 Gmelin, Tryon considers as a separate sub-genus, and names it Gitocentrum (1862). 



17. Zirphcea, Leach, 1847, (H. and A. Adams, Gen. ii, p. 327). Shell 

 oval, without accessory valves, hinge plate not reflexed, the umbonal muscle only 

 covered with a horny epidermis, anterior hiatus always open. Through the want of 

 accessory valves Zirphma no doubt forms a passage to the Gastroch^nid^, 



The first reliable records of fossil species of fkolabinm are from the lower 

 mesozoic strata, (triassic and Jurassic), though, as I have already noticed, traces 

 of their borings in fossil- wood and in stone have been found already in carboni- 

 ferous beds, and some of these hollows most likely have been excavated by molluscs 

 belonging to this sub-family. In the cretaceous strata we find a number of forms 

 which in their general character much resemble the recent species, and this resem- 

 blance becomes still more apparent in the forms found in the tertiary beds. 

 Several of the miocene species, like Teredo Norvegica or Xylophaga dor sails, are 

 for instance, not distinguishable from those still found living. Tryon enumerated 

 (in 1867) 63 recent species of fholadin^, 



Pictet (Mat. Pal. Suisse, 3"^^ part., 1864, p. 26, etc.,) gave a list of the cretaceous species 

 of Pholas, taking the signification of the genus in the old sense. He enumerates the following 

 European species :— 



1. — Ph. icaunensisj Cott. 



^.—Ph. Eoemeri, d'Orb., 1850, (Prod. II, p. 72, Fistulana constricta, Romer), has the 

 form of a PholacUdea, or a Parapholas ; the posterior end is not sufficiently attenuated for 

 a Martesia. Dr. J. Miiller in his Suppl. to the ^^ Monograph, d. Petr. der Aachner Kreidef.,^^ 

 (1859, p. 16,) also proposes for Romer's F. constricta the name Ph, Roemeri, which d'Orbigny had 

 anticipated. 



3. — Ph. prisca, Sow., is evidently a Martesia, boring in wood. 



* Letcconyx.TL. and A. Adams, is based upon the internal spathulse of Pholas costata, (see Proc Zool Soc 

 Lond., for 1865, p. 754). 



