Ill A I A 



t<> the narrowly rounded po-terior extremity. Antorior eml neatly rounded. \s 

 tlian tlio posterior. Ventral margin gently convex in the nii<l'll>, more strongly and 

 almost e.|iiall\ em-M-l at the ends. An obscure umhonal ridge traceable from the 



I three-fourth- of the distance to the posterior basal edge. Above it an 

 impressed narrow line, beyond which the -urface descends rapidly to the donal 

 margin, i a-t- of tlie interior with a narrow, slightly curved, clavicular impre ion 

 ju-t in front of the beaks, extending but little more than one-third of the di-tam-e 

 lie antero-basal margin. Surface of casts with a few obscure growth lines or 

 fold.-. Point of greatest convexity a little above and behind the center of the shell. 

 In a dorsal view the central half of the outline is very slightly flattened. 



Length. 17.2 mm.; bight. '..0 mm.; thickness of both valves, 5.8 mm. 



This shell appears to be related to C. cunealus and C. elongatus, described by Hall 

 from the Silurian rocks of Nova Scotia (Can. Nat and Geol., vol. ">. pp. 14$ and 1-V>. 

 IsOO). It is, however, specifically distinct, the shape being different and the poste- 

 rior sinus situated higher up and very much less defined. C. pliinululus (Conrad) and 

 C. ellipti<-n* Urieh, also have somewhat different outlines and have the cardinal 

 slopes less abrupt, the whole surface, in those species being more uniformly and less 

 convex. 



formation and focaftfy. -Middle Catena t. ft. MlnneftoU, wlu-iv II IN atMorlaUxl with Cttno- 



itcmtn intermedia. 



iiopiiottrri NKULKcrrs Hull. 



I'l.ATK XI. II KKJd. -. 



laphorwt tir a lrcfu II A I.I, I *-. Oil. Sur. Win., Tol. 1, p. 6i. (Fltfiintl l.ut ti.t d,~--r; 

 Compare Clidophonu < .Vu. -,,l,it fabxla HALL, 184.1. AIH.T. Jmir. Sri. and Art*, vol. xllll, p. 2B6. 



Shell varying greatly in size, the smallest observed having a hight of on! 

 mm., with a length of 5 mm., while in the largest seen (from Graf, Iowa,) these 

 dimensions are respectively $.*> mm. am! !' mm.: thickness of the latter about 

 6 mm. 



Shell transversly subelliptie. rather strongly convex; ends subequalty rounded, 

 the anterior generally a little narrower than the posterior; the outline of the latter, 

 however, often exhibits a tendency to become angular just beneath the middle and 

 obliquely subtruncate above; basal and dorsal margin broadly convex. Beaks small, 

 somewhat tumid, placed about one-third of the length of the shell behind the ante- 

 rior extremity. Surface marked by fine concentric lines and several stronger varices 

 of growth; the latter show through the shell so as to be visible on casts of the inte- 

 rior. Hinge plate narrow, not over half the length of the shell, minutely toothed; 

 denticles twenty or more in each valve, three-fourths of the nun ng post. 



