1919] 



Davidson: A Cestraciont Spine from Nevada 



435 



its length was probably less than half as great. The greatest width 

 of the Idaho specimen is 23 mm. and that of the Middle Triassic spine 

 12 mm. The shape of the spine, proportion of ornamented and un- 

 ornamented surface, arrangement of tubercles and type of their 

 sculpturing are very different. Cosmacanthus elegans has a strongly 

 marked anterior enamel keel which is absent in the Nevada specimen. 



This spine resembles Asteracanthus ornamentissimus Agassiz 5 in 

 the absence of an anterior keel. In size, however, as well as in general 

 form, distribution and detail of ornamentation, the two spines are 

 unlike. 



In general appearance it resembles Geisacanthus bullatus St. John 

 and Worthen 6 from the Chester limestone, Chester, Illinois. This, 

 however has a strong enamel keel and the part represented as orna- 

 mented is greater in extent. The shape of the inserted portion differs 

 from that of the Nevada spine. The sculpturing of the tubercles is 

 similar, although their size and arrangement are not the same. 



5 Agassiz, L., Becherches sur les poissons fossiles, III Atlas, tab. 7, figs. 11, 

 13, 14, 15, 1843. 



6 St. John, O., and Worthen, A. H., Descriptions of fossil fishes, Palaeontology 

 of Illinois, Geol. Surv. Illinois, vol. 6, pt. 2, pp. 446-447, pi. 17, figs. 3, 4, 1875. 



Transmitted November 28, 1917. 



