250 PALAEONTOLOGY OF ILLINOIS. 



important character, not so clearly discernible in the fragment of 

 spine upon which the existence of the genus was based, of the back- 

 ward curvature, which removes it from the Drepanacanthi, with which 

 it bears some resemblance in the character and disposition of the 

 longitudinal ribs and tubercles occupying the exposed parts. Unfor- 

 tunately the entire section of the spine is not preserved, so that we 

 are still uninformed in regard to the contour of the posterior face. 

 Specifically allied to the Kinderhook Glymmatacanthus Irishii, St. 

 J. and W., it is readily distinguished by the more rapidly tapering, 

 smaller, transversely elongate, and much more prominent or conical 

 tubercles, which are also more closely ranged along the costae, and 

 more widely spaced laterally than observed in the unique example 

 of the earlier species just mentioned. 



Geological position and locality: Keokuk limestone; Government 

 canal excavations around the Keokuk rapids on the Mississippi, 

 above Keokuk, Iowa. 



GLYMMATACANTHUS PETRODOIDES, St. J. and W. 



PL XXV. Fig. 2. 



A mere fragment of an ichthyodorulite, hardly a centimetre 

 square, in the collection of Dr. Hambach, is probably referable to 

 the above genus. The superficial ornamentation is so well preserved 

 as to enable very satisfactory comparisons with other species of the 

 genus, as also to show its distinctive features so far as they depend 

 on the character of the surface ornamentation. The fragment rep- 

 resents a bit from the lateral walls inclosing the pulp cavity, and 

 probably pertained to a spine of medium size. The longitudinal 

 costae show only moderate convexity with the usual elongate irregu- 

 lar punctae ; closely arranged along their crests are rows of strong 

 conical tubercles which crowd the surface, and which are deeply 

 graven with sharp, strong carinee, radiating from the apex and con- 

 stricted at the base. The latter character, which, on account of the 

 resemblance to the vertical plication of the cones of Petrodus, sug- 

 gests the specific designation, also offers a marked and tangible 

 contrast with the tubercular ornamentation observed in connection 

 with the previously described species. 



Geological position and locality: Chester limestone; Chester, 

 Illinois. 



