434 CEETACEOUS PELECYPODA 



Nilsson appears to have been the first to distinguish two species of very similar, 

 roundly ovate, shape ; the one with numerovis arcuate striiB or ribs, and the other 

 with a much smaller number of very similar ribs ; the former* he identified with 

 Sowerby's P. arcuat us, {which is a Jurassic species), and the latter he named 

 P. vlr(jatus. 



Omitting some doubtful references of various authors to Nilsson's supposed 

 m'cuatus, we find the species again under Geinitz' name ciiroatus, and shortly after- 

 wards d'Orbigny figured and described it in his Pal. fran9. under the name of 

 virffcitus of Nilsson. Forbes, when noting the Indian form, very probably based 

 his identification upon d'Orbigny's figure, and in so far was quite con-ect, but 

 that figure represents Geinitz' curvatus and not Nilsson's cb-yatus. Reuss 

 recorded a right valve of this species under the old name arcnatus, and a left 

 one, to all appearance belonging to the present species, he called d'waricatus. 

 Geinitz in his " Quadersandstein-gebirge" very properly separates his cunatiis 

 from true virgatus, but his reference to the synonymy of the latter species is not 

 correct, as I shall note further on. D'Orbigny, when suggesting the name siih- 

 virgatus for the Indian form, justly remarks that it diifers from Nilsson's virgatus 

 by much finer ribbing. This difference equally applies to the form described by 

 Zittel from the Gosau deposits under Nilsson's name. 



Zittel complains of the want of accuracy in Nilsson's original figure of 

 virgatus for which there is no reason, because that species is always much coarser 

 ribbed than curvatus, even allowing for the slight difference in the ribbing of the 

 two valves of the latter species, a character which should never be overlooked. 

 Goldfuss' arcuatus from Aachen is undoubtedly the same species as Nilsson's 

 virgatus, as I have had opportunity of comparing original specimens from those 

 localities. It is the only reliable record of true virgatus I know of; most of the 

 other references quoted under virgatus probably belong to curvatus. 



Localities. — Arrialoor and Kaudoor, in whitish sandstone, (Arrialoor group) ; 

 north of Garudamuugalum, and near Veraghoor, in a bluish and grey sandstone 

 (Trichinopoly group) ; rare. 



Formations. — Arrialoor and Trichinopoly groups. There is no difference to be 

 observed between the specimens from the two series of beds. 



In Europe the species occurs in the senonien beds near Aachen, in the lower 

 and middle Pla?ner of Saxony and Bohemia, in the Gosau deposits of the Austrian 

 and Bavarian Alps, in the Turonien in Erance, &c. It is not, however, reliable to 

 refer to foreign localities, because the distinction between virgatus and curvatus has 

 not always been noticed. Coquand records virgatxs from his " etage Mornasien" 

 ( ? = Turonien) from the Province Constantiue (Palcont., p. 299), and as he 



* Reuss (Bojlim. Kreide., 184G, pt. ii, p. 28, pi. xsxix, fig. 8,) identifies this form with one from Bohemia under 

 the new name P. concentrice-piiiictaliis, which is nearlj- circular, and, as regards shape, therefore, very diiferent from 

 N ilsson's figure of his supposed arcuatus. I douht that both are the same, in spite of Xilsson's statement that the 

 shell of his species is very thin, resembling that of Amus. membraiiaceum, for the thickness of tlie valves varies 

 greatly both in true virgatus and in curvatus, and the right valve is often a little thinner than the left one. 



