534 THE PALEONTOLOGY OF MINNESOTA. 
[Cyrtodonta, 
The Cyrtodontide seem to me to be a family of shells that is essentially Lower 
Silurian, the Upper Silurian species being both few in number and of small size 
and thin-shelled. Indeed the evidence at hand goes to show that the family 
became practically extinct with the close of the Upper Silurian. [If this is true 
then we cannot very well ally them with recent families of shells, and as they con- 
stitute an easily recognized group of genera it has been deemed necessary to estab- 
lish a new family for their reception. 
The Cyrtodontide, despite the well developed dentition prevailing among the 
typical members, seem to represent a very early type of structure, and one that 
probably antedated both the Ambonychiidae and Modiolopside, to which also they 
appear to be more closely related than to any other of the contemporaneous 
families. Thus certain of the earliest species of Vanuxemia (e. g. V. terminalis) 
greatly resemble true Ambonychia, while the majority of the Modiolopside present, 
aside from the hinge, an internal conformation of parts that is decidedly like the 
prevailing appearance in the present family. Perhaps the only constant difference 
between the shells of these three families is that while those of the Ambonychiide 
and Modiolopidw were provided with a well develeped epidermis those of the 
Cyrtodontide preserve no trace of such a covering. 
Genus CYRTODONTA, Billings. 
Cyrtodonta, BrLLiInGs, 1858, Can. Nat. and Geol., vol. 3, p. 431. 
Palearca, HAut, 1859, Pal. N. Y. vol. iii, p. 27; also 12th Rep. Reg. N. Y. Mus. Nat. Hist., p. 10. 
Angellum, S. A. MILLER, 1878, Jour. Cin. Soc. Nat. Hist., vol. i, p. 105. 
Cypricardites, HALL, and most American authors, (not of CONRAD). 
Shell varying from transversely or obliquely ovate to subcircular, moderately 
ventricose. Beaks prominent, rather tumid, incurved, situated in the anterior third, 
fourth or fifth of the shell. Surface marked with concentric lines of growth. No 
lunule nor escutcheon. Hinge plate strong, nearly straight, often with a narrow 
and not sharply defined ligamental area. Cardinal teeth well developed, subequal, 
generally obliquely curved, sometimes nearly horizontal, two to four in each valve, 
situated mostly in front of the beaks. Posterior lateral teeth usually two or three in 
each valve, strong, elongate, more or less curved and slightly oblique, situated near 
the extremity of the hinge. Adductor muscular scars placed immediately beneath 
the two sets of teeth, both subovate, the posterior very faint, the anterior only 
moderately impressed. Pallial line simple. 
Types: C. rugosus and C. canadensis of Billings. 
This is an excellently defined genus and one of the largest of the paleozoic 
genera of Lamellibranchiata. It is also pre-eminently a Lower Silurian genus, the 
