728 DEEP WATER MOLLUSKS AND BBACRIOPODS—DALL. voL.xvn. 



This is tlie most common and variable species of tlie ^N'orthwest coast 

 and attains a notable size, especially in the deep, quiet waters of Puget 

 Sound. The original transversa was described from a rather smooth 

 specimen, while Gould's type was strongly radiately ribbed. Dr. David- 

 son would use both names in a varietal sense, retaining the older for the 

 species, but the grades of variation are so numerous that it is doubtful 

 how far this would be justified, as it sometimes happens that one-half 

 the shell is smooth and the other half ribbed. One feature, however, 

 is constant in all the multitude of specimens I have examined; the 

 flexure of the middle of the valves, though often feeble, is always 

 concave in the brachial valve and convex in the pedicle valve. The 

 young specimens from Monterey, Catalina Island, and other southern 

 localities are frequently suffused with reddish or crimson about the 

 margin and on the backs of the ribs. The ribs, when well developed 

 and normal, vary from 18 to 40, bifurcating or intercalary toward the 

 margin. The specimen figured is young, and chosen because it is of the 

 same width as the specimen of T. occidentalism with which it is intended to 

 be compared, and also is, for the species, very strongly ribbed. These 

 specimens are often found near extreme low- water mark, but it is proba- 

 ble that they never attain their full growth in such situations. The 

 completely adult specimens appear confined to deeper water.' They 

 sometimes reach the size of 50 mm. high and ~j'i mm. wide, Avith a 

 diameter of 31 mm. This species is figured in the Proceedings of the 

 Academy of Natural Sciences of Philadelphia for' 1891 on pi. iv, 

 figs. 6 and 7, but unfortunately the numbers in the text are 8 and 9, 

 wliich refer to the figures of T. occidentaiis. As in the text referred to, 

 the attempt was made to separate the present species from T. occiden- 

 talism which had become confused with it, this was doubly unfortunate, 

 hut as the writer saw no proof of the plate it was beyond his power to 

 remedy. The present refiguring, it is hoped, will finally clear up the 

 confusion. 



This species is easily distinguished from T. occidentaiis in the adult 

 state if any attention be given to the diagnostic characters, but there 

 are others from which it is less sharply distinguished. From Dallina 

 grayi, Davidson, of Japan, the southern reddish si^ecimens of transversa 

 externally are with difticulty distinguished. In fact, one might fancy 

 that the two species were descendants of one ancestor, which, for some 

 leason, in Japan continued its evolution to the Dallina stage, while 

 those in America ceased when they arrived at the Terehratalia stage. 

 D. yrayi in the adult stage has hardly any flexure, and in the variety 

 transversa^ Davidson, the flexure is double, but in the young, and in 

 such of the adults as show the flexure clearly, the two medial riblets 

 are convex in the brachial valve, com])lemented in the pedicle valve by 

 a corresi>onding depression. In Terehratalia transversa^ Sowerby, the 

 flexure is wider, and the converse of what occurs in B. grayi. There 

 is another Terehratalia in Japan with much the same sculpture as T, 



