LAMELLIBRANCHIATA. 541 
Crytodonta parva.] 
Cyrtodonta halli Nettleroth, sp., from the upper beds of the Hudson river group of 
Kentucky, is shorter and thicker, and has more prominent beaks and umbonal 
ridge. 
Formation and locality —The typical form occurs in the upper part of the middle third of the Tren- 
ton shales, six miles south of Cannon Falls, Minnesota. The Galena variety was collected near Wykoff, 
in Fillmore county. 
CYRTODONTA, PARVA, ”. Sp. 
PLATE XXXIX, FIGS. 24 and 25. 
This small species seems to be closely related to C. affinis Ulrich and C, halli 
Nettleroth, sp. In some respects it is intermediate between those species, differing 
from the first in its greater convexity, stronger umbonal ridge and larger anterior end, 
these being points in which the shell agrees rather closely with the latter. From both 
it differs in the more abruptly rounded postero-basal margin and straighter ventral 
outline. 
Adductor scars very faint, undetermined. The specimen being a cast of the 
interior, the detail of the hingement could not be made out with certainty. 
Greatest length, 9.5 mm.; greatest hight, 6.5 mm.; thickness, 4.5 mm. 
Formation and locality—Middle Galena, near Fountain, Minnesota. 
CYRTODONTA ROTULATA, 2. Sp. 
PLATE XXXIX, FIGS. 16—19. 
Shell small, moderately ventricose, nearly erect, the outline uniformly rounded 
(subcircular) except at the dorsal margin, which is straight behind the beaks and 
somewhat insinuated in front of them; hight and length about as five is to six; 
posterior extremity of hinge angular. Beaks small, incurved, scarcely prominent, 
situated about one-fourth of the length of the shell behind the most prominent point 
on the anterior margin. Umbonal region full, but not excessively so; point of 
greatest convexity a little above and in front of the center of the valves; postero 
cardinal slope gently concave, causing this part of the shell to appear as slightly 
alate. Surface marked with fine concentric lines, with a few (those shown in the 
illustrations) stronger than the rest. Shell and hinge plate thin; dentition undeter- 
mined beyond this that it is essentially as called for by the genus. Muscular scars 
unknown. 
Iam not acquainted with any species of Cyrtodonta, described heretofore, with 
which this one might be confounded. C. persimilis Ulrich, a much larger species, is 
in outline somewhat like it, but on comparison proves to have the beaks situated 
farther forward and to be proportionally less ventricose. Several species belonging 
