200 bulletin: museum of comparative zoology. . 



that has come to light, and analogy with this form leads us to expect in ad- 

 vance of the anterior dental plate a series of Helodus-like teeth on either side 

 above and below, and in front of these at the symphysis in at least one jaw, a 

 series of bilaterally symmetrical teeth, arched in a single plane, and corre- 

 sponding to the form described by Newberry as Helodus coxanus.^ We are not 

 yet in possession of adequate material, however, to attempt a theoretical resto- 

 ration of the dentition of Deltodus, and the final solution of the problem must 

 await the discovery of naturally associated parts. 



In order that students may observe for themselves the nature of the material 

 upon which the above identifications and conclusions are based, several speci- 

 mens of Deltodus teeth are figured in the accompanying plates. In Plate 4, 

 Fig. 38, is shown a small-sized but very perfect example of the posterior dental 

 plate of D. occidentalis ; in Plate 5, Fig. 53, a specimen of the form correspond- 

 ing to the so-called D. latior St. J. and Worth, which we regard as a variety 

 of D. occidentalis transitional between that species and L). sjxUtdatiis ; and in 

 Plate 4, Figs. 41, 42, are shown two rather large-sized examples of the posterior 

 dental plate of D. spatulatus, one from the Burlington, and one from the 

 Keokuk beds. Finally, in Plate 5, Fig. 55, a very excellent example is repre- 

 sented of the anterior dental plate of I), spatidatus, according to our inter- 

 pretation of the so-called Deltodopsis? convolutus St. J. and W. As to 

 the size attained by the posterior dental plates of D. spatulatus^ we can only 

 affirm that no specimens are known exceeding that figured by Newberry and 

 Worthen under the name of D. alatus, but one of almost equal proportions is 

 preserved in the Museum of the State University of Iowa at Iowa City. 



Formation and Locality. — Kinderhook. Burlington, and Keokuk Groups ; 

 Iowa, Illinois, and Indiana. 



Deltodus occidentalis (Leidy). 



(Plate 4, Fig. 38; Plate 5, Fig. 53.) 



1857. Cockliodus occidentalis J. Leidy, Trans. Amer. Phil. Soc. (2), Vol. XI., p. 88, 



PI. v.. Figs. 3-16. 

 1866. Deltodus stellatus Newberry and Worthen, Pal. Illinois, Vol. II., p. 97, PI. 



IX., Fig. 2(uon Fig. 3?). 

 1883. Deltodus occidentalis St. John and Worthen, 0}>. cit., Vol. VII., p. 150, PI. IX., 



Fig. 9 {non Fig. 10). 

 1883. Deltodus latior St. John and Worthen, Ibid., p. 145, PI. IX., Figs. 11, 12. 

 1883. Deltodus intermedins St. John and Worthen, Ibid., p. 153, Pi. IX., Figs. 14, 15. 

 1897. Deltodus complanatus J. S. Newberry, Trans. N. Y. Acad. Sci., Vol. XVI., p. 



298, PI. XXIV., Figs. 1-7. 



The teeth referred to this species exhilnt a wide range of variation, and 

 while the more common expressions are ([uite distinct, there are arclied forms 

 like the type of the so-called " D. latior " which appear to connect the species 



1 Trans. N. Y. Acad. Sci., Vol. XVI, 1897, p. 301, PI. XXIV., Fig. 24. 



