VEETEBEATES. 453 



first mentioned may prove to be identical, their differences attributable 

 to difference in age. 



Compared with forms from other horizons, the first named, Ph. pro- 

 clvcits, bears perhaps closest resemblance to the Keokuk form which we 

 have indicated under the name Ph. parvulus, but is apparently less 

 robust and relatively shorter. That above referred to under the name 

 Ph. depressus equally resembles the medium size individuals in the 

 Upper Burlington fish-bed, which we believe to be identical with Ph. 

 gigas, IS. and W., the only appreciable difference consisting in the rela- 

 tively less strong and laterally more compressed figure of the present 

 spines. The sharply carinated anteroinferior shoulder of the last 

 described form, Ph. carinatus, distinguishes it from any form with which 

 we have had the means of instituting a comparison. 



Physonemus pabvtjxtjs, St. J. and W. 



PI. XVIII, Fig. 11, 12. 



This little spine is distinguished by the strong curvature and propor- 

 tionately stout build, the transverse section showing gently convex 

 lateral faces which rapidly converge towards the anterior edge, the pos- 

 terior face relatively broad, anteroposterior shoulder moderately promi- 

 nent, gently convex above, and reaching probably half way across the 

 base posteriorly, below which the inserted portion is compressed, slightly 

 notched in front, and terminated in an obtuse point. The largest speci- 

 men is a little less than one inch in length, about .35 inch in breadth at 

 the basal line, and about .15 inch across the posterior face in the lower 

 part of the exposed portion. 



We have seen only a couple of specimens of this form, both of which 

 are denuded of every trace of external ornamentation, and the posterior 

 walls are worn away, the posterior aspect being deeply channeled by 

 the pulp cavity. One of these specimens, belonging to the collection of 

 Dr. Williajis, of Boouville, exhibits nearly the entire outline and the 

 basal region ; the other, obtained by Mr. Worthen, though less perfect, 

 shows the transverse section of an individual of about the same size. 

 Its stout proportions, strong curvature, and thick transverse section, 

 distinguish it from Ph.proclivus, the young of Ph. gigas, IS. and W., as 

 also Ph. Chester ensis. 



Position and locality : Eare in the fish-bed horizon of the Keokuk 

 limestone ; Boonville, Missouri, and near Warsaw, Elinois. 



