312 



In its perfect condition the tooth has approximated I4 inches in length, 

 and about 1 inch in breadth at l)ase. 



A simihar tooth from the coal-measures of Illinois has been described 

 under the name of Cladodus mortifer by Professor Newberry, in the second 

 volume of Worthen's Geological Survey of Illinois, published in 18G6. Mr. 

 Orestes St. John has likewise described some teeth of the same species from 

 the coal measures of Nebraska, in the Proceedings of the American Philo- 

 sophical Society for 1870, and in Hayden's Report on the Geological Survey 

 of Nebraska, published this year. 



XYSTRACANTHUS. 



Xystracanthus arcuatus. 



A second cartilaginous fish of the Coal-period is indicated by a remarkable 

 dorsal spine, discovered by Messrs. Meek and Hayden in the Upper Carbon- 

 iferous rocks of Leavenworth City, Kansas. The specimen, represented in 

 Fig. 25, Plate XVII, lies partially imbedded in a piece of yellowish lime- 

 stone, also containing a few minute crinoid segments. The point of the spine 

 and its root of insertion are destroyed, and the specimen is otherwise muti- 

 lated and appears somewhat crushed, but it is sufficiently characteristic to 

 distinguish it from ichthyodorulites previously described. 



The spine is strongly curved, appears flattened at the sides, and is rounded 

 at tlie borders. Its transverse section is narro\V ovoid, with the narrower 

 extremity toward the convex border. The spine is longitudinally striated, 

 and in its present condition the bone is brown and quite friable. The sides 

 and concave border of the spine are furnished with white, shining, enamel- 

 like tubercles of various sizes. The smaller ones are half ovoid ; larger 

 ones are conical or half conical ; and the largest, which occupy the upper and 

 lower part of the concave border, are crescentoid, and embrace the latter. In 

 shape and attachment the larger tubercles remind one of minute polypori 

 projecting from the stem of a tree. They are convex above, and flat, or 

 slightly concave, below. 



PETALODUS. 

 Petalodtjs alleghaniensis. 



Petalodus is another extinct genus of cartilaginous fishes, allied to our 

 living sharks, which was originally characterized by Owen, and was also 

 established on isolated teeth from the Carboniferous formations of Europe. 



