42 CEETACEOUS PELECYPODA 



or perhaps more probably a Cnjptomyaj in which case it would be the first species of the genus from 

 cretaceous rocks. 



59-64. — Meek (Smiths. Misc. CoU.^ No. 177J quotes from North America N. alaformis, fibrosa j 

 Moreauensis and ventricosa, and Gabb described N. dolahrmformis (Pal. California^ i;, 1864;, p, 153). 



65-66. — Two species occur in our South Indian cretaceous deposits^, N. mtttua and detecta, the 

 last of which has some resemblance to Corh. striaUda^ but its shelly of which traces are present^ is 

 very thin. The first species is from the Trichinopoly^ the second from the Ootatoor group. 



POROMYA, 



67-69. — I am acquainted for the present only with the three Indian species^ P. glohu- 

 losa, lata, and sujoerha (see p. 47^ &c.) . The two former were, without any apparent reason^ transferred by 

 D''Orbigny to Lyonsia. Forbes also considered the Corh. cBquivalvis of Goldfuss as a Poromya, 

 but according to the hinge it does not belong to this genus^ though the surface is partially provided 

 with minute grooves. I shall describe it under Fholadomya, and state the necessary reason for this 

 alteration subsequently. 



Lutraria nuciilceformis , Schafh._, (Bayern^s Leth. Geog.^, p. 175^,) I would be inclined to 

 consider as identical with our P. glohulosa, at least the general form of both is the same. The form, 

 figured by Schafhseutl, loc. cit., (pi. 41, fig. 2,) as Corhda im]pressa (1863, non idem, Eichwald 

 Leth. Ross., liv. xi, 1867, p. 741,) is probably also a Poromya. 



I am not certain whether Zittel^s Panop. freq^uens from i\iQ Gosau (Denksch. Akad., Wien, 1865, 

 XXIV, pt. ii, p. Ill, pi. 1, fig. 5,) does not also belong to the present genus. The form and the general 

 character of the shell are almost identical with those of our P. lata, but the hinge is very similar 

 to that of a Parwjjma, or rather to that of Pleuromya ; it has, however, like Paromya, one tooth in 

 each valve and posterior to it a groove, in which, in the last genus, the cartilage is situated. 

 Whether that was the case with the species in question, Zittel's figure does not show clearly, for 

 the tooth in the left valve is placed too much anteriorly, almost more so than that of the right 

 valve, behind which it ought to fit when the valves are closed. The most important distinction 

 to be observed in ZitteTs figure, is the isolation of each of the teeth along the lower margin, 

 while in Poromya the inner hinge area is thick, and the teeth and pits generally appear on it as 

 projections and grooves. In specimens of our collection the hinge-teeth could unfortunately not be 

 made visible, and the shell surface is also not well preserved. — I expect several of D'Orbigny's 

 Panojma will be shown to belong to Poromya; in some, like P. i^?^^?a><2(?^^>, the author describes 

 the granulation of the surface, similar to that of Poromya, but quite distinct from that of Panojma. 



I have already mentioned Corhula oitusa, which Bosquet considered to be a Poromya. Ceromya 

 recens, Pkoladomya aptiensis, and fallax described by Coquand in his Monograph of the Fossils of 

 the Etage Aptien of Spain are more likely Poromyce than Ceromyce, I am also by no means certain 

 whether such species as Pliolodomya Banctce-crticis , or Ph. Valangiensis of Pictet and Campiche, 

 ought not to be more correctly referred to Poromya than to Pkoladomya. 



Judging from external appearance and the imperfectly known hinge, I suppose Corhula gigantea, 

 Sow., (Min. Conch., vol. iii, p. 13, pi. 209, figs. 5-7,) is also a Poromya. It very much resembles 

 our Por. superba, differing from it by tlie anteriorly extended portion of the shell. The species 

 has been placed by Morris in the genus Thetis, that author having considered Forbes"' Poromya 

 as identical with Thetis, which opinion is, as I have already stated, untenable. Pictet and 

 Campiche (Pal. Suisse, 4th Ser., 3^^^ p^^^ p^ 2IO,) are of opinion that the species probably 

 belongs to a new genus; it is said to possess the same deep sinus, as Thetis has, but Sowerby does 

 not say anything on this point; unless, therefore, Pictet and Campiche have examined fresh 

 specimens, the question would still remain unsettled. 



These are only a few of the most prominent instances indicating the occurrence of Poromya in a 

 fossil state. I could add a much greater number of suggestions regarding the generic determination 

 of the cretaceous species, but it would be hardly fair to go into these details without the examination 



