142 



concentric ribs, which are usuall}- much wider than the deep, narrow 

 grooves between them. On the kniule and escutcheon the ribs are small, 

 narrow and acute, and commence at the superior border both in front 

 and behind as simple raised lines. 



The specific characters of this shell are very nearly the same as those 

 of a Mactra from the Cretaceous rocks of Southern India, described by 

 Professor Forbes under the name Mactra tripartita, G. B. Sowerby, MSS.* 

 Stoliczka says that the central area of M. tripartita is divided from the 

 lunule by a narrow groove, and from the escutcheon by a brojui, shallow 

 sulcus, neither of which are to be seen in any of the specimens collected 

 by Mr. Richardson, but these markings are not mentioned in Forbes' 

 description of M. tripartita, nor are they indicated in either of Stoliczka's 

 figures of that shell. 



• Mactra (Cysibophora ?) Warrenana, Meek & Hayden. 



Plate 17, figure 9, and plate 19, figures 3 and 3a. 



Mactra Warrenana, Meek & Hayden. — Proc. Ac. Nat. Sc, Phil., Vol. VIII. (1856), 



p. 271. 

 Mactra (Cymbophora) Warrenana, Meek. — Rep. Inv. Cret and Tert. Foss. U. Miss. Co., 



p. 208, pi. 30, figs. 7, a, b, c, d. 



Collected by Mr. Richardson at the same localities and from the same 

 divisions as the preceding species, also at Newcastle Island, and two 

 miles and a half up the JSTanaimo River, Y. I., in Division A. 



The (Bpeoimens have been compared with authentic examples of M. 

 Warrenana from Dakota, received from Mr. Meek, and no appreciable 

 differences could be perceived between them. 



The present shell may be the same as the smooth form of C. Ashburnerii, 

 but Mr. Gabb's descrij^tion of the latter embraces such a wide range of 

 variation that thei'e are few species of Mactra from the Cretaceous rocks 

 of North America to which it would not apply. Conrad claimsf that 

 two species have been confounded together under the name C. Ashburnerii, 

 one of which is strongly ribbed, and the other marked only by fine lines 

 of growth, but he is certainly mistaken in the supposition that the latter is 

 an Eocene fossil. In the Vancouver Cretaceous these smooth Mactras may 

 be distinguished at a glance from the large and coarsely ribbed variety of 

 C. Ashburnerii, with which they are almost invariably found associated, by 

 their smaller size, more decidedly triangular outline, and, more especially, 



♦ Transactions of the Geological Society of London, Series II., Vol. VII., p. 142, pi. 15, Ag. 17. 

 t American Journal of Concliology, Vol. I., p. 364. 



