448 PALAEONTOLOGY OF ILLINOIS. 



Genus PHYSONEMUS, Agassiz. 



The first authentic notice of the genus Physonemus, though first recog- 

 nized by Prof. Agassiz, (Poiss. Foss., Ill, appended list of new and 

 undescribed forms,) we owe to the investigations of Prof. McCoy, who 

 introduced a diagnosis of its generic characteristics in his work on the 

 British Palaeozoic Fossils, p. 638, wherein he notices the supposed origi- 

 nal form referred to by Prof. Agassiz, Ph. subteres, together with the 

 description of a species new to science, Ph. arcuatus, p. 638, PL 3 1, fig. 29. 



From the description above referred to, the spines were apparently 

 regarded as having a posterior curvature, reversing the relative posi- 

 tion of parts, by which the convex posterior border was supposed to 

 constitute the anterior margin. Later, Messrs. Newbeery and 

 Woethen, in a notice of a remarkable species belonging to the Upper 

 Burliugton limestone, which they described in a preceding report of the 

 Illinois Geological Survey, (Vol. IV, p. 373, PI. II, f. 1,) under the 

 name Ph. gigas, correctly interpret the position or forward curvature of 

 the apex of the spine ; and though comparison is made with Ph. arcua- 

 tus, McCoy, of the Irish Carboniferous limestone, no reference is made 

 to the generic diagnosis preceding the description of the latter form. 



The material accessible to us, through the labors of Messrs. Springee, 

 Wachsmuth, Van Hoene and ourselves, though not extensive, 

 embraces more or less satisfactory representation of the several forms 

 afforded by the various formations of our Lower Carboniferous group, 

 first appearing in the Kinderhook or lowest member, and extending up 

 into the Chester or upper division, from which latter a single small 

 specimen has come to our notice. With this material we have been 

 enabled to arrive at a pretty thorough understanding of the generic 

 peculiarities as represented in the several species now determined, and 

 from which the following amended description is presented: 



Dorsal spine generally strong, laterally compressed, deeply imbedded, 

 curved with the apex directed towards the front. Exposed portion of 

 the spine more or less flattened laterally, truncated posteriorly, rounded 

 in front along the concave anterior margin which arises from a more or 

 less prominent or laterally expanded shoulder ; lateral faces occupied 

 by parallel longitudinal costte which bear sculptured tubercles whose 

 apices are directed downward, and form a pair of perhaps less promi- 

 nent costse, on either side of the anterior edge arise a row of similar 

 but much larger, alternately disposed tubercles, which extend at least 

 to the anteroinferior shoulder. Posterior face depressed (?) or convex 

 and traversed by a more or less prominent median keel. Pulp cavity 

 relatively large, extending well towards the tip, and opening out into a 



