VERTEBRATES. 457 



reversed position of the apical crest in tbe large transverse tubercles 

 along the anterior margin, tbe inferior declivity gently descending and 

 tbe superior slope abrupt, as shown in fig. 5 b, enlarginent showing the 

 slightly eccentric middle row and the two large lateral rows immedi- 

 ately occupying the auterior margin. The line of insertion is very 

 oblique, indicating a correspondingly depressed or moderate inclination 

 of the spine. 



A specimen from the same horizon at St. Louis, representing an 

 apparently young spine, -fig. 6 a, may prove to be specifically identical 

 with the above, having the same general outline and showing the basal 

 characteristics of the genus, or the inconspicuous antero inferior shoul- 

 der and general slender, elongated outline, but in a nude condition 

 without trace of tuberculose ornamentation. 



Position and locality : Rare, upper beds of the St. Louis limestone ; 

 Alton, Illinois, and St. Louis, Missouri. 



Genus XTSTBACANTHTTS, Leidy. 



Xystracanthus arcuattis, Ltedt, Proc. Acad. Nat. Sci. Phila., 183!), 3; Estinct Vert. Fauna, Dr. Hay- 

 den'9 U. S. Geol. Surrey of tbe Territories, 187;), p. 312, P:. XVII, fig. 25. 



The above referred to genus and species was recognized by Dr. Leidy 

 from a unique and nearly eutire spine, discovered by Messrs. Haydbn 

 and Meek in the Upper Coal Measures at Leavenworth, Kansas. From 

 the solitary example, which does not preserve the inserted extremity, 

 it would be difficult to clearly define the characteristics of the spine, 

 but which are quite apparent in the light of other and generically iden- 

 tical specimens which have since been discovered in the Coal Measure 

 strata of Illinois. On application, Dr. Leidy kindly loaned us the 

 original specimen noticed by him, a comparison of which with the Illi- 

 nois spines conclusively establishes the generic identity of the latter with 

 the Kansas specimen. 



Allied to Physonemtts, Agassiz, from which it is mainly distinguished 

 by the more slender, straighter outline, and the less preponderance of 

 the anteroinferior shoulder, it remains to be ascertained in what essen- 

 tial feature the spines of this genus differ from Drepanacanthus, N. and 

 W., occuring in the Lower Carboniferous formations. Presenting nearly 

 the same proportionate characters, perhaps slightly more rapidly taper- 

 ing, it would appear that we must look for distinguishing features, if 

 such exist, in the style and arrangement of the external ornamentation. 

 But as far as our observations extend, there would appear to be quite 

 as great variableness in this respect amongst the Coal Measure forms 

 as there is between them and the Lower Carboniferous forms. In view 



—59 



