210 



BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



noticed its resemblance in ornamentation to that of P. gemmatus from the same 

 horizon. The specimens figured by St. John and Worthen are likewise from 

 the distal third of the spine. The total length, as shown by one or two good 

 examples in Iowa City, is really much longer than these authors supposed, and 

 the breadth nearly equals that of P. gemmatus. The proportions of the two 

 species are in fact very similar, the chief differences consisting in ornamental 

 details, form of cross-section, and the greater development of tubercles along 

 the concave border in the present species. In the distal portion of the spine 

 these tubercles frequently appear as strong acuminate denticles. The pulp- 

 cavity remains open for a short distance on the convex side near the base, and 

 continues close to this side throughout the spine until near the tip. Like the 

 preceding species, it is extremely probable that the spines of P. stellatus were 

 curved forwards. 



Formation and Locality. — Keokuk limestone ; Iowa and Illinois. 



LIST OF NORTH AMERICAN SPECIES OF PHYSONEMUS. 



ERISMACANTHUS M'Cot. 



The two European and one American species of this genus that have been 

 described are evidently very closely related to the type of Physonemus, but 

 differ in that the spines are divaricated, the two branches extending in opposite 

 directions in the same vertical plane. The imperfect Ichthyodorulites known 



