Otenoaontan.edlaUs.1 LAMELLIBRANCHIATA. 593 



This species is distinguished from C. levata Hall, sp., by its shorter form, abruptly 

 truncated posterior end and subalate antero-cardinal region, and in the posterior 

 instead of anterior position of the beaks. In casts of the interior the beaks are also 

 smaller and more prominent. C. abrupta Billings, is more ventricose, longer and not 

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



Formation arid locality.— GooA specimens of this species are exceedingly rare, but illy preserved 

 casts of the interior, whichare provisionally referred here, are not uncommonly associated with O. soeialis 

 in the middle third .of the Trenton shales at Minneapolis and other localities in the state of Minnesota. 



Ctbnodonta medialis, n. sp. 



PLATE XLII, FIGS. 50-52. 



I 



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

 C. scofieldi. From the first it differs 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- 

 cardinal 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 and appears relatively wider. From C, scofieldi it differs in having the 

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

 ity, the umbonal ridge 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 differentations leads me to believe that the form under consideration is more 

 closely related to C. nitida than to C. scofieldi. Perhaps it would be sufficiently dis- 

 tinguished 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. No. 8311) from the "Lower Blue" limestone at Janesville, 

 Wisconsin. In these specimens there is an antero-cardinal wing as in C. nitida, but 

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

 midlength instead of behind. C. leveta Hall, sp., also seems to belong here, but it is 

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

 have been subjected to a critical examination. 



Formation and locality.— Middle third o"f the Trenton shales, Minneapolis and near Cannon Falls, 

 Minnesota. A cast of the interior from the Galena shales of Goodhue county, probably belongs here. 



Ctenodonta scofieldi, n. sp. 



PLA.TB XLII, FIGS. 53-58. 



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

 rounded in front and below, narrow behind, with small, prominent, incurved beaks, 



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