I. \MKI.1. II. I! \N< III .VI \ MM 



t'ii*-'. 



None of the northwestern .-penmen- (if this species soon ly me quite reach . 

 length of 4d nun., the average beiiiK about _'" mm. In Canada they grew to much 



;ter -i/e, some of the specimens from I 'a i lunette's Rapid- mi tli> nttaua ' 

 having ii length of more than GO ram. 



A ociated with thi- t>pMMI in \\I---OIIMII ami at 1'auquett's Rapid- there 

 form which, though it ha- Loon identified unreservedly with C. tuisuta by Hall and 



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

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

 of the midlength. an- a t rifle behind that point, the muscular impressions are deeper, 

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

 'lie. This form, for which I propose the varietal designation robust a, was figured by 

 I'rof. Mall in the Tenth Annual Report of the Regents of the University of New York 

 on page 183 as T>IHn<>may nasuta. He figures two specimens of which the smaller 

 may belong to nasuta. The larger example, however (figures 1 and 8), I refer to the 

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

 possession at thi- 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 Kent ncky 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 thi- Kentucky species the bight is even a trifle greater, the base is not 

 Minute, 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. natuta occurs >|iarlnt{ly lo the lowei Trenton limestone at Minneapolis 

 and in the n>Ul<ll<- third of th>- Tr.-ui-n -hales In Guodhue county, Minnesota. In Wisconsin the specie* 



' abundant in th>- "Lower Blue" and the ' I'ppcr Buff" limestone* at Belolt, Janenvllle and Mln- 

 r;ii Point, it has also been found in th<> -a mi- beds at IMxmi :iiul oth>-r localities in Illinois. In Canada 

 It occurs in the Black River and Trenton llni.vttone* at Ottawa and numi-nni* oth.-r |*>lnU. The ordinal 

 types of the species came fmtn thf Trcnfm limestone at Mlddlevllie and Tn-tit<.n K.iil-. N.'w York, and 



it.iNu.-d by Prof. J. M. Safford among the fo&slN .,; in- "C, ntra r>>ek ~ 



lonex In Tennessee. Variety rolnuta o i':i.i. ( u.-t t.-'n l!.ipi<K n>:u < ittawa, Canada, nnd Ii 



i-r Ruff limestone at Belolt, Wisconsin. 



M*t Reg. N<. -.ii:. var. robiu-i 



XODONTA SI'BNASITV. I, 

 i-i.ATr. xi. 1 1 nun. M-X 



Tlii- -hell is no doul>t closely related to C. nasiita. Imt. aside from its much 

 smaller dimensions, it ditters in sevenil particular^ that have seemed of -ntlicient 

 importance to merit -pecitic recognition. Thu- the posterior end is somewhat 

 longer, the beaks being placed farther in front of the middle, the anterior en 

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

 the lower margin of the hinge plate is almo '"ad of biconvex, while 



