VERTEBRATES. 409 



latter form. We have a few imperfect teeth from the same formation, 

 at Pella, which are probably referable to this species. Tbe latter speci- 

 mens, however, are very small and somewhat peculiar in the truncation 

 or broadly rounded condition of the apical extremity, although in other 

 particulars they are not dissimilar from the Alton specimens. 



At the localities mentioned, an anomalous form occurs, which we have, 

 provisionally, r.eferred to a distinct species, under the name P. distortus, 

 the affinities of which are noted in detail under that head. As there 

 stated, their "relation to the present form was suggested by their resem- 

 blance to abnormally developed individuals of the form common in the 

 Upper Burlington limestone, P. striatus of Messrs. Kewbebby and 

 "vYobthen, but it is exceedingly difficult to reconcile the variations 

 presented by these teeth with the marked distinctive characteristics of 

 the form described above, though to 'a much less degree when they are 

 compared with P. pseudosagittatus. 



The present form is most nearly allied to that originally recognized by 

 Prof. Agassiz under the name P.psittacinus, from the Carboniferous lime- 

 stone of Armagh, Ireland. It differs, however, from the European form 

 in the less tumid condition of the basal region, and also the less hori- 

 zontal direction of the basal band of the convex crown face. However, 

 in the collection of the Museum of Comparative Zoology, there is a 

 single specimen of the European form, in which the basal band is gently 

 curved upward and downward on approaching the lateral angles in the 

 convex crown-face, much in the same manner, though in a less degree 

 than in the present form, instead of being perfectly horizontal as repre- 

 sented in the figures given by Prof. McCoy in the British Palaeozoic 

 Fossils, PI. 3 1., fig. 14. 



Position and locality : Upper beds of the St. Louis limestone ; Alton, 

 Illinois, and Pella, Iowa. 



Genus PELTODUS, tf. and W. 



This genus was founded upon a species from the Upper Coal Mea- 

 sures, which was described in Vol. IV of this Eeport (pp. 302, 363, PI. II, 

 fig. 7, 7 a,) under the specific designation P. ungui/ormis, N. and W. 

 From the notes appended to the generic diagnosis above cited, it would 

 appear that the genus was regarded as intimately allied to the Cochlio- 

 donls, or "intermediate in character between Psammodus and Cochliodus." 

 The material now possessed, perhaps, more clearly illustrates the affini- 

 ties of the genus, and which has suggested the following observations 

 on the homologous features which it possesses in common with the Peta- 

 lodonts, to which the genus belongs. The general figure of the teeth is 

 that characteristic of the Petalodonts, consisting of a flattened crown, 



—53 



