20 CRETACEOUS PELECYPODA 



5. JouanneHa,'Desh., 1828, (H. and A. Adams, Gen. ii, p. 329). Shell more 

 or less globose, sculptured as in Xylophaga, inequivalve, front gape closed in the adult 

 state ; a single accessory valve is present. Besides a few recent species there are 

 some very similar forms known from the tertiary beds, and I shall allude to one or 

 two from tbe cretaceous strata. 



5ffl. Fholadopsls, Conrad, 1849, (Proc. Phil. Acad., p. 156, &c.,) is stated to 

 differ from the last in having a " subcentral, radiating" groove. 



6. Martesia, Leach, 1824 (H. and A. Adams, Gen. ii, p. 330). Shell generally 

 clongately ovate, j^osteriorly produced, equivalve ; anterior hiatus closed ; one um- 

 bonic accessory valve; one narrow stilet-like post-umbonic plate, and a similar 

 postero-ventral one ; valves sculptm-ed similarly to those of Xylophaga. 



This genus is well known from cretaceous as well as from tertiaiy deposits. 



6a. Pholameria, Conrad, 1865, (American Journ. Conch., i, p. 2), has been pro- 

 posed for an oligocene species, described in Journ. Acad. Nat. Sc, Phil., 2nd Ser., i, 

 p. 127, pi. 13, fig. 3, vinder the name of Fholas triquetra. The shell has the form of 

 a sbort Martesia, but without accessory plates ; nothing else, however, occurs in the 

 specific description which would indicate any peculiarity to justify the fonnation of 

 a new genus. 



7. Schroteria, Tryon, 1862 (Proc. Phil. Acad., 1862,) has one pre-imibonic 

 plate ; tbe anterior hiatus is probably closed in the adult shell ; the type is Pholas 

 cordata, Schroter. 



8. Diplothyra, Tryon, 1862, (ibid). Shell like in Martesia, but with one larger 

 umbonic and one smaller pre-imibonic accessory valve. Stimpson (American 

 Journ., XXXV, 1863, p. 306,) states that the species described by Tryon as 

 D. Smithll is a true Martesia. 



9. Farap)liolas, Conrad, 1848, (Proc. Acad. Nat. Sc, Phil., p. 121). Shell 

 ovately elongated like Martesia, equivalve, sculptvired at the sides similarly as in 

 Xylophaga; anterior hiatus closed ; umbonal accessory valve longitudinally divided 

 in two, often extending backwards ; superior margin in front of the umbones 

 reflexed ; median area generally with two transverse fvirrows from the umbones. 



The forms called Farapholas are very closely allied to those of Martesia, and 



they could probably be considered merely as sub-generic to the latter genus. The 



division of the umbonal plate does not seem to be always perfect, as I shall have 



occasion to notice subsequently when describing a cretaceous species. Paraph. 



mersa. However, there is a peculiar distinctive character to be noticed in a few of 



the Indian species which in external form and in the two sulci on the median area of 



the shell perfectly agree with Conrad's Parapholas, except that their umbonal 



valves are short and broad. I refer here specially to three forms which I have 



examined: Martesia fliiminalis, Blf., (Jour. Asiat. Soc, Bengal, 1867, vol. xxxvi, 



p. 67, pi. 3, figs. 1-3, — or a very closely allied species) ; a new species apparently 



with the anterior hiatus permanently open, and boring in wood along the mouths 



of rivers of the Orissa coast, and a third (typical) species from Ceylon. All these 



have in the left valves a distinct claw-shaped tooth, and an obsolete tooth in the 



