22 CRETACEOUS PELECYPODA 



orieulalis, 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-umlional ; 

 hinge plate reflcxed over the beaks and closely applied. 



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



clear. 



16a. Tryon separates the species with a wide emarginated anterior hiatus (like 

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



16. Dactilina, Gray, 1847, (H. and A. Adams, Gen. ii, p. 325,) 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 D. dati/lus. 



16a. 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 Gitocentriim (1862). 



17. Zirp)h(sa, Leach, 1817, (H. and A. Adams, Gen. ii, p. 327). Shell 

 oval, Avithout 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 Zirphcea no doubt forms a passage to the Oastrocr^nidje. 



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

 mesozoic strata, (triassic and Jurassic), though, as I have ah-eady 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 

 Avhich 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 dorsalis, are, 

 for instance, not distinguishable from those still foimd living. Tryon eniunerated 

 (in 1867) 63 recent species of pholadin^. 



Pictet (Mat. Pal. Suisse, S""' 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 

 Eurf)pean species : — 



1. — Ph. icaunerisis, Cott. 



2. — Ph. Roemeri, d'Orb., 1850, (Prod. II, p. 72, Fistnlana constricta, Romer), has the 

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

 a Martesia. Dr. J. Miiller in his Suppl. to the "Monograph, d. Petr. dor Aaehner 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. 



* Leuconyjc, H. ami A. Adams, is based upon the internal spathulje of Pholas costata, (see Proc. Zool. Soc., 

 Loud., for 1865, p. 754). 



