454 PALEONTOLOGY OF ILLINOIS. 



Physonehus Altonensis, St. J. and W. 



PI. XIX, Fig. 1-3 



Fin-spine attains large size. External portion erect, sublenticular in 

 transverse section, rapidly tapering to the apex which is posterior to a 

 vertical median line,hight less than the horizontal dimension of the base, 

 anterior margin regularly curved and forming one-fourth of a circle, merg- 

 ing below into tbe laterally expanded, gently convex, elliptic antero- 

 inferior shoulder, beneath which the base forms a sharp constriction ; 

 lateral surfaces flattened above, very slightly convex below, gradually 

 converging to the obtusely angular or rounded anterior margin, more or 

 less compressed inferiorly along the posterior borders, broadly expanded 

 along the basal line, the denuded surface finely striato-punctate. Pos- 

 terior face very gently and somewhat irregularly arched, occupied by a 

 prominent median keel and gently channeled either side, postero lateral 

 angles well-defined above, inferiorly obsolete or merging into the body 

 of the spine. Base sharply compressed beneath the anteroinferior 

 shoulder and deeply notched in front, produced anteriorly iuto a long, 

 slender point, posteriorly merging into the exposed body of the spine, 

 and similarly striato-punctate, the channel of the internal cavity reach- 

 ing to the extremity. Internal cavity large, extending nearly to the 

 apex, lenticular or long-ovate in transverse section, the obtuse extrem- 

 ity anterior and encroaching on the anterior wall towards the apex 

 which is even slighter than the dense body along the posterior borders, 

 and deeply excavating the postero-iuferior margin. A large specimen 

 about seven inches in length, measures two inches in greatest diameter, 

 .60 inch in thickness, basal line about 4.25 inch, elevation of the exter- 

 nal portion over the horizontal line continued from the shoulder above 

 three inches. 



All the specimens of this species examined by us are imperfect in the 

 nonpreservation of the external ornamentation, of the special character 

 of which we have no intimation. The collection of Mr. Yan Hokne 

 affords a fine, large individual exhibiting the entire outline of the spine, 

 and perfect with the exception of the tuberculose ornamentation ; and 

 from the same locality (Alton), Mr. Van Horne has obtained a smaller 

 specimen, which latter is in the condition of the majority of the speci- 

 mens of the genus, that is, the denuded external surfaces and the bro- 

 ken posterior face. Together, these specimens contribute important 

 data to our knowledge of the actual outline of the ichthyodorulite and 

 the modifications produced by abrasion, and which is very similar in all 

 forms. Thus the imperfect spine above referred to scarcely affords a 

 suggestion of the entire outline as it is exhibited in the large specimen, 



