510 DR WHEELTON HIND ON 



Observations. — There are several specimens of the shell in the collection labelled 

 Woodland Point, one only from each of the other localities. That from the Star-fish 

 bed is crushed, and it is possible that I may be wrong in referring it to the same 

 species. The Thraive specimen seems to agree in all details with those from Woodland 

 Point. The ornament and general contour agree typically with Hall's diagnosis of the 

 genus, and I have no hesitation in referring them for the present to Paracyclas, until 

 further details of the interior are available for study. 



Cyrtodonta, Billings, 1858. 



Gyrtodonta, Billings, 1858, Canad. Nat. and GeoL, vol. iii. p. 431. 

 Palsearca, Hall, 1859, Pal. N.Y., vol. iii. p. 270. 



„ ,, ,, 12th Rrp. Regents New York JMus. Nat. Hist., p. 10. 



,, Salter, 1866, Mem. Geol. Survey Gt. Brit., vol. iii. p. 341. 

 Angellam, Miller, 1878, Jour. Cincinnati Soc. Nat. Hist., vol. i. p. 105. 

 Cyrtodonta, Ulrich, 1897, Geol. Minnesota, Fin. Rep., vol. iii., pt. ii., p. 534. 



Generic Characters. — Shell transversely ovate or subcircular, gibbose, umbones 

 swollen, placed at the anterior third of the hinge line. Dorsal slope compressed. 



Interior. — Hinge plate of 3-5 oblique teeth in front, becoming smaller as they pass 

 backwards, and sloping forwards ; elongate, slightly diverging teeth behind, thin in 

 front, but becoming thicker as they pass backwards. Anterior adductor scar shallow, 

 triangular, placed beneath the hinge teeth, remote from the margin. Posterior 

 adductor scar shallow and large, placed below the posterior hinge teeth, with hollow of 

 the dorsal slope. Pallial line simple. 



Exterior.— The surface is ornamented with concentric lines and rugae of growth. 



Observations. — There has been a good deal of obscurity as to the synonomy of this 

 genus, the whole history of which is given by Ulrich in the final report of the geology 

 of Minnesota (op. supra cit.). I agree with him that Billings' name should stand. 

 Hall evidently agreed that his Palsearca must fall, but thought that Cypricardites, 

 Conrad, should take its place. This genus, according to Ulrich, was insufficiently 

 described, and the hinge described as characteristic of Cypricardites does not correspond 

 to that of Cyrtodonta. Ulrich goes on to say, p. 536 (op. supra cit.) : " He (Conrad) 

 represents the cardinal teeth as diverging from the beak, much as in a Lyrodesma, and 

 says that the anterior one is the 'largest and most prominent.' Neither of these 

 conditions is ever present in Cyrtodonta. On the contrary, the teeth are subparallel, 

 and to be called horizontal rather than radial, while the anterior one, if any can be so 

 called, is the smaller." This criticism only adds to the difficulty, and makes it question- 

 able whether Ulrich is dealing with shells of the character of Billings' species, the more 

 so when on p. 533 he, in discussing the relation of Cyrtodonta to the Arcidse, and with 

 Macrodon in particular, says : " I should hold that Macrodon was not a member of 

 Arcidse, since that genus most certainly did not arise in Ctenodonta." The last 

 sentence expresses a fact which no one will dispute, but there is no connection at all 



