LAMELLIBRANCHIATA. 593 
OCtenodonta medialis.] 
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 and locality—Good specimens of this species are exceedingly rare, but illy preserved 
casts of the interior, which are provisionally referred here, are not uncommonly associated with C. socialis 
in the middle third of the Trenton shales at Minneapolis and other localities in the state of Minnesota. 
CTENODONTA MEDIALIS, ”. Sp. 
PLATE XUII, FIGS. 50—82. 
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.—Middie third of 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, ”. sp. 
PLATE 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, 
—38 
