LAMELLIBRANCHIATA. 585 
Ctenodonta subnasuta.] 
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 it 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 Tellinomay nasuta. He figures two specimens of which the smaller 
may belong to nasuta. The larger example, however (figures | 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” limestones at Beloit, 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 River 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. Safford 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 Reg. No. 8317; var. robusta, 8315. 
CTENODONTA SUBNASUTA, 2. 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 
