_. I.AMKI.UHKAV in \ ;,'.i:t 



Ot*oodul mo.llll. | 



This species is distinguish,^! from C. levaia Hall, sp.. by its shorter form, abruptly 

 trunrut.',! posterior ..... I an.l stil>ulnt. aiitonv, -animal region, and in the posterior 

 in>toad of anterior position of the beaks. In casts of the interior the beaks are also 

 -mailer ami more prominent. C. nbntpta Hillings, is more ventricose. longer and not 

 so high anteriorly. The two wpecies next described are more closely related. 



Formation .m-i i,-,iiity. <;..od specimens nf this species are exceedingly rare, but Illy preferred 

 casto of the Interior, which arc provisionally referred here, are nit unc-.numonly antedated with C. toeiatit 

 In tii- m..| i .. thinl i.f Hi.- Tr. -nt. .11 shales at Minneapolis and other localities In the state of Minnesota. 



' 'TKNODOXTA MKDIALIS, n. sp. 

 PLATE XLII. riOB. o-as. 



This species seems to occupy an intermediate position between C. nitida and 

 C. trori'/'li. 1'rom the first it diflers in having the beaks situated about midway 

 between the extremities, the posterior end longer, more oblique and more narrowly 

 rounded at the extremity, and the anterior end shorter and blunter in the antero- 

 cardiual region. The posterior part of the back is wider, because the umbonal ridge 

 is somewhat stronger and extends farther downward. Finally, the hinge plate is 

 more curved an.l appears relatively wider. From C. xcofieldi it differs in having the 

 beaks centrally situated instead of one-third of the length from the anterior extrem- 

 ity. the umlional ri<lge less sharp, the posterior end shorter and wider, and the hinge 

 plate stronger and more numerously denticulate. A careful estimation of the value 

 of the ditlt'ientations leads me to believe that the form under consideration is more 



ly related to C. nitidn than to C. scofieldi. Perhaps it would be sufficiently <li- 

 tingnished as a variety of the former. 



Another form of this type is represented by two casts of the interior in the Sur- 

 vey collection (Mus. Reg. Np. 8311) from the "Lower Blue" limestone at Janes vi lie. 

 WiM-oiiMii. In these specimens there is an antero-cardinal wing as in C. nilida, but 

 the posterior end is too long for that species, the beaks being slightly in front of the 

 mi'llen-th instead of behind. C. Itvjfta Hall, sp., also seems to belong here, but it in 

 not safe to say anything positive about that species till the original New York types 

 have been .-injected to a critical examination. 



Formation and locality. Mi<l<li" thinl ( tin- Tr> nton -hale*, Minneapolis and near Cannon Falls. 

 Minnesota. A cast t th" int*Tir fnun the Galena shales of Ooodhue county, probably belongs here. 



CTRNODONTA SOOKIKLDI, n. sp. 



I-I.\TK xt. ii nos. m-m. 

 Shell small, strongly convex, transversely somewhat amminate ovate, broadly 



roumled in front aiul In-low, narrow behind, with small, prominent, incurved beaks, 

 -38 



