PTYCHODUS. 



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Family Myliobatidj;. 

 Genus PTYCHODUS, Agassiz. 



Ptychodus, L. Agassiz, Poiss. Foss., Feuill., 1835, p. 54 (name only), and vol. iii, 1839, p. 150. 



Avlodus, F. Dixon, Geol. Sussex, 1850, p. 366. 



Sporetodus, E. D. Cope, Bull. "U.S. Geol. Surv. Territ., i, no. 2, 1874, p. 47. 



Hemiptychodus, O. Jaekel, Eocanen Selaehier Monte Bolca, 1894, p. 137. 



Generic Characters. — Teeth quadrate in shape, with more or less elevated 

 crown, which overhangs to some extent and is separated from the root by a 



H&, 



I 5 ? w 



r V^ 



^3 



Wljho 



Fig. 69. Ptychodus ; diagram of arrangement of teeth in upper (A) and lower (B) jaws, o, middle row ; 



i — vi, paired lateral rows. 



constriction ; crown enamelled, and marked with transverse or radiating ridges, 

 surrounded by a more finely marked marginal area of variable width ; attached 

 surface of root smooth, not grooved. Teeth arranged as a close pavement in 

 longitudinal rows on the symphysial portion of the jaws (Text-fig. 69) ; a 

 median row in one jaw (probably upper) of relatively small teeth, flanked by 

 several symmetrical pairs of rows which diminish in size laterally ; the median 

 row in the opposing jaw (probably lower) comprising the largest teeth, and the 

 flanking paired rows gradually diminishing in size laterally. Vertebral centra in 

 the form of well-calcified, short, biconcave discs, each strengthened by delicate 

 concentric laminae. 



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