

!.\MU.|.IM;\N. 111 \ i ,, 



oftheshHl leaving a d.-tmrtdepressi,,,, , thecaste. Muscular ncaw large. Ml.. 

 .il-ut mi.lway in tin- po>ten, car Imal half ,,f the valve, the t v, 



k. the upper .,ii.. ,,val in -hape ami about ,,n,. -third ;w large M the i- 

 nearly circular lower one. 



Tin- p,M.-i-i..r extremity i> more |.m.li. i more narrowly oim-d than ii, 



"'' to tins genus. 



i loralii,,. l^, w , r limestone of the Trenton formation at Minneapolis Minn. 

 Mia. Keg. No. 5530. 



UNHATA l-'.mmon*. 



> a. 



in-..;. ll.-iK.rt K, p. aw. 



JtabMfeMa Mutate HAU., 1847. i , ., r ic, vol. i, p. 165. 



Shell snbijnadrate. canlinal margin long, straight, anterior side strai^lit, ni-aily 

 \.-rti.:al. curving sharply Itackward below into the gently convex base, which in it- 

 turn < iirves rapidly upward into the broadly rounded posterior margin; ant 

 cardinal angle about 85 , postero-cardinal angle about 115". BeaU prominent, 

 attenuate. >li^htly incurved, with the umbones strongly convex, the anterior -lope 

 alirupt. the rapidity of the descent becoming gradually less in following the 

 margin around to the posterior extremity of the hinge, where it in very gei 

 -animal -lope conca\e. becoming strongly so and very abrupt in noaring the lx- 

 Surface marked with broad concentric folds, which are strongest on the cardinal 

 and miilional slopes and fade away gradually in curving around to the anterior >idc. 

 Immediately beneath the beaks the anterior side of a good cast of the interior 

 I a -harply defined lunule-like impression, which, having been occupied by 

 an internal thickening of the margin of the valves, wag scarcely indicated on the 

 exterior of the shell. Hinge plate narrow, muscular impressions undetormii 



The above description is based upon the specimen illustrated on plate x.x 

 It presents no evidence of distortion and seems to be in every respect in a good 

 .ition. Compering thfe eXMUple with Mall's description and figures 

 of the New York type- of the species we observe that it differs in several lar 



that mi-lit be regarded as important. The outline is more nearly quad rate, and the 

 convexity of the valves less, giving a form that deviates from the figures of the 



New York -pet-mien precisely a> ' . rrectn does from ' '. InmeUosa. Hall also mentions 

 the absence of a "definite lunette." while such an impression i- ly present 



in the casts of the Minnesota specimens. Despite these difleren I am almost 

 contident of the spe<-itic identity of the latter and the tyjHjs of the species, because I 

 am inclined to doubt the actual existence of the discrepancies noticed. 



