466 University of California Publications in Geology [Vol. 8 



ably wider than the ridges themselves, but on the adult shell the inter- 

 spaces become narrower toward the base until the larger ridges are so 

 crowded as to be indistinct. Hinge plate heavy ; left valve with long, 

 heavy nymph plate, on top of which and transversing it longitudinally 

 is a distinct ridge. Posterior cardinal over two-thirds the length of 

 the nymph plate, which latter is the same height as the cardinal, being 

 separated from it only by a shallow groove; the outer edge of the 

 cardinal and inner edge of the nymph plate show evidence of having 

 been rugose. Middle cardinal about half as long as posterior cardinal, 

 the space between the two being extraordinarily broad and deep. An- 

 terior cardinal thin, about the same length as the middle cardinal, the 

 two lying almost parallel with each other. Anterior lateral long and 

 heavy, the top broadly rounded, the upper end connecting with the 

 anterior dorsal margin. Right valve with a long, heavy nymph plate, 

 which becomes thinner toward the lower end and is rugose internally. 

 Posterior cardinal long, heavy, bifid, about two-thirds the length of 

 the nymph plate and separated from it by a deep, elongate socket. 

 Middle cardinal heavy, rather short and higher than the others. An- 

 terior cardinal very thin, shorter than the middle cardinal and situ- 

 ated close to it; anterior lateral pit deep. Posterior dorsal edge of 

 right valve below the ligamental area grooved to receive the cor- 

 responding edge of the left valve. Pallial sinus ample, slightly ascend- 

 ing and pointed in front. The impressions of two internal ridges are 

 seen on the cast of the shell, which ridges are characteristic of the 

 section Amiantis. 



A. dalli is very closely related to A. eallosa Conrad, a Recent species 

 on the West Coast, showing all the minor characters described by Dall 

 as being characteristic of the section Amiantis. (For description of 

 genus Amiantis and section Amiantis by Dall, see Transactions of the 

 Wagner Free Institute of Science, vol. 3, pt. 6, pp. 1257-P258.) As 

 far as the writer is aware, this makes the second species belonging to 

 this genus described in North America. A. dalli in comparison to A. 

 eallosa is higher in proportion to its length and the concentric undula- 

 tions are more irregular. The liuiule and escutcheon of the two species 

 are very similar. The hinge plates, however, differ considerably. The 

 nymph plate on the left valve of the latter is longer but not so high 

 or so heavy and it lacks the longitudinal ridge seen on top of the 

 former. On A. dalli the middle cardinal of the left valve is separated 

 by a broader socket than on A. eallosa. On the former the middle and 

 interior cardinals lie nearly parallel, while on the latter they diverge at 



