474 PALAEONTOLOGY OF ILLINOIS. 



dinal rows and diagonal series in both directions, vertically spaced by 

 about tbeir own diameter, closely approximated laterally, largest and 

 most crowded in front and occupying the convex margin, apparently 

 scattered posteriorly, abruptly truncated above with an arched trans- 

 verse crest, sloping below and more or less distinctly radiatingly stria- 

 ted, though generally quite smooth. The costae do not appear to be 

 distinctly defined ; the vertical interspaces beneath the tubercles are 

 finely striated, or striato-punctate. The posterior or concave edge set 

 with rather strong, laterally compressed, slightly downward curved 

 denticles, spaced by little more than their diameter, and rapidly dimin- 

 ishing in size interiorly. A specimen, probably imperfect, half an inch 

 in length, measures about .20 inch across the expanded base. 



The form described is known from only two specimens, one of which 

 is quite perfect so far as relates to its external features, though the 

 basal portion is doubtless broken away. It is most intimately allied to 

 G. Jatus of the Keokuk limestone, both in shape and general ornamen- 

 tation ; the only marked distinguishing feature being the less strongly 

 sculptured tubercles and their more crowded arrangement. 



In case the specimens noticed represent nearly the entire length 

 attained by the spines, of both these forms, one can scarcely fail to 

 recognize affinities which almost equally ally them with Pnigeacanthus — 

 their chief distinction, assuming their basis of support to be the same, 

 consisting in the occurrence of a row of strong denticles in the concave 

 margin. 



Position and locality : Upper beds of the St. Louis limestone ; St. 

 Louis, Missouri. 



Gampsacanthits ! latus, St. J. and W. 



PI. XXII, Fig. 14. 



A fragment of a spine, discovered by Dr. G. A. Williams, showing 

 about half an inch of the upper extremity of a medium-sized specimen, 

 seems to be allied generically with the form noticed under the name 

 Gampsacanthus typus. The specimen is characterized by its somewhat 

 rapidly converging anterior and posterior margins, terminated in an 

 obtuse point, laterally compressed, broadly rounded in the gently arched 

 convex margin, lateral faces apparently slightly converging to the more 

 sharply rounded concave margin, which is occupied by a row of very 

 strong, slightly compressed and recurved denticles, which are smooth or 

 perhaps faintly ridged vertically, widely spaced and apparently dimin- 

 ishing in size below, where they may assume much the appearance of 

 the tubercles. The flattened or very gently arched lateral faces and 

 convex border present numerous scute-like, coarsely stellate tubercles, 



