534 THE PALEONTOLOGY OF MINNESOTA. 



[Oyrtodonta. 



The Cyrtodontidce 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 Cyrtodontidce, 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 AmhonycUidae and Modiolopsidce, 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 Modiolopsidce 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 Ambonychiidce 

 and Modiolopidce were provided with a well developed epidermis those of the 

 Cyrtodontidce preserve no trace of such a covering. 



Genus CYRTODONTA, Billings. 



Cyriodonio, Billings, 1858, Can. Nat. and Geol., vol. 3, p. 431. 



Palcearca, HALL, 1859, Pal. N. Y. vol. ill, p. 27; also 12th Rep. Reg. N. Y. Mus. Nat. Hist., p. 10. 



Angellum, S. A. Miller, 1878, Jour. Gin. Soc. Nat. Hist., vol. 1, p. 105. 



Cypricardites, Hall, and most American authors, (not of Conkad). 



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 



