LAMELLIBRANGHIATA. 585 



Ctenodonta subnasata.] 



None of the northwestern specimens of this species seen by me quite reach a 

 length of 40 mm., the average being about 25 mm. In Canada they grew to much 

 greater size, some of the specimens from Pauquette's Rapids on the Ottawa river 

 having a length of more than 60 mm. 



Associated with this species in Wisconsin and at Pauquett's Rapids there is a 

 form which, though iib has been identified unreservedly with C. nasuta by Hall and 

 others, I find to be not strictly identical with that species. The anterior end is higher 

 and larger, and the posterior end shorter, so that the beaks, instead of being in front 

 of the midlength, are a trifle behind that point, the muscular impressions are deeper, 

 and the hinge plate is on the whole narrower and much less constricted in the mid- 

 dle. This form, for which I propose the varietal designation robusta, was figured by 

 Prof. Hall in the Tenth Annual Report of the Regents of the University of New York 

 on page 183 as TelUnomay nasuta. He figures two specimeiis of which the smaller 

 may belong to nasuta. The larger example, however (figures 1 and 3), I refer to the 

 variety robusta, and I do so with the utmost confidence, the specimen being in my 

 possession at this moment. At Pauquette's Rapids the variety attains about the 

 same size as the typical form of species, but in Wisconsin it-is much the larger. 



Near the top of the Trenton in Kentucky there is a form, that I shall call C. regia, 

 which seems to represent the culmination of the differentiation begun in the variety 

 robusta. In this Kentucky species the hight is even a trifle greater, the base is not 

 sinuate, the muscular scars are very deep, and the hinge plate stronger than in both 

 the variety and the typical form of nasuta. 



Formation and locality.— C. nasuta occurs sparingly In the lower Trenton limestone at Minneapolis 

 and in the middle third of the Trenton shales in Goodhue county, Minnesota. In Wisconsin the species 

 is more abundant in the "Lower Blue " and the " Upper •Buff" limestonea.at Belolt, Janesville and Min- 

 eral Point. It has also been found in the same beds at Dixon and other localities in Illinois. In Canada 

 it occurs in the Black Eiver and Trenton limestones at Ottawa and numerous other points. The original 

 types of the species came from the Trenton limestone at Middleville and Trenton Falls, New York, and 

 it is catalogued by Prof. J. M. SafEord among the fossils of his " Central," "Glade" and "Carter's Creek " 

 limestones in Tennessee. Variety robusta occurs at Pauquette's Rapids near Ottawa, Canada, and in the 

 "Upper Buff" limestone at Beloit, Wisconsin. 



Mus Beg. No. 8317; var. robusta, 8315. , 



Ctenodonta subnasuta, n. sp. 



PLATE XLII, FIGS. 34-36. 



This shell is no doubt closely related to C. nasuta, but, aside from its much 

 smaller dimensions, it differs in several particulars that have seemed of sufficient 

 importance to merit specific recognition. Thus the posterior end is somewhat 

 longer, the- beaks being placed farther in front of the middle, the anterior end is 

 more obtuse in a dorsal view, the beaks are turned anteriorly rather than backward, 

 the lower margin of the hinge plate is almost straight instead of biconvex, while 



