Evans.] 



A Neir Cestraciont Spine. 



399 



truncated top. On the upper third of the spine, the tubercles are disposed 

 in longitudinal rows parallel with the anterior edge. They increase in 

 number below and dispose themselves in perceptibly oblique rows, tending 

 to radiate from the central portion of the posterior edge. The medullary 

 cavity extends for about .85 of the length of the spine and opens in a wide 

 and deep furrow on the lower end of the posterior face. The edges of the 

 lower end of the furrow are sharp and slightly incurved, becoming rounded 

 as they approach each other above to meet near the upper end of the 

 inserted portion. 



AFFINITIES. 



Isolated dermal spines closely resembling the above in many 

 particulars occur at numerous horizons, being most common on 

 this continent in the Devonian, Sub-Carboniferous, and the Coal 

 Measures. They have been usually grouped under the general 

 term "Ichthyodorulites " and are most frequently referred to the 

 Cestraciont idae. 



Among the Ichthyodorulites, a great diversity of form and 

 ornamentation exists, but many spines can be found having some 

 characters in common with the Paris specimen. The smooth 

 base, ornamented faces, and the internal cavity with a low 

 posterior opening, are characters possessed by many genera. 



Of the European genera, Asteracanthus Agassiz is near this 

 spine in general form and ornamentation, but the similarity 

 breaks down on examination of the individual tubercles and of 

 the posterior surface. One of the conspicuous differences is seen 

 in the entire absence of enlarged tubercles, denticles or teeth on 

 this spine, whereas it possesses a sharply defined enamel keel, 

 which is not present in Asteracanthus. 



The tendency to arrangement of the tubercles in transverse 

 rows is a character perhaps most marked in Oracanthus Agassiz. 

 This genus, however, never shows truncation of the posterior 

 border and the tubercles are also distinctly different from those 

 of the Paris spine. 



The general shape of the Paris specimen and its cross-sections, 

 present a strong similarity to the figures of NemacantJius motii- 

 lifer Agassiz* from the Triassic of England, but the presence of 

 a well marked postero-lateral row of denticles as also the character 

 of the lateral tuberculatum in Nemacanthns preclude its refer- 

 ence to that genus. 



* Agassiz L. Recherches sur les Poissons Fossiles. Ill Atlas 1843. Tab. 7, 

 Figs. 11, 13, 14, 15. 



