VERTEBRATES. 235 



they have a pectinated appearance at their edges, in token of the 

 tubercles originally ornamenting their crests. They are so crowded 

 upon the costae as to compel an alternate disposition ; in the an- 

 terior portion they are crowded vertically, but over the greater por- 

 tion of the surface they are spaced by about their own diameter. 

 The pulp cavity is relatively large at the inferior angle of the pos- 

 terior face, beneath which it first opens out, exposing the deep 

 trough-like excavation extending thence to the proximal extremity, 

 but rapidly diminishing above where it is filled by the peculiar, dep- 

 osition of dense matter analogous to the inferior homogeneous layer 

 met with in the associated teeth of Cochliodonts, the 50 or 60 mili- 

 metres of the distal extremity being solid, 



Mr. Butters has obtained from the St. Louis formation a single 

 representative of the above described species, representing a spine 

 20 to 25 centimetres in length, of which the posterior face consti- 

 tuted half, little more or less. Unfortunately, the specimen was 

 much mutilated in freeing it from the limestone matrix, both 

 extremities being broken, and preserving a length of about 15 cen- 

 timetres of the middle portion of the spine. It is a fine representa- 

 tive of a peculiar form of ichthyodorulite, of which several examples 

 are known, and which is distinguished by the great fore and aft 

 extent of the dorsal line and corresponding breadth of the inserted 

 basal portion. In the foregoing description, allusion was made, in 

 connection with the latter character, to the probable recumbent 

 position the spine occupied in the creature's back; but this, after 

 all, may not necessarily indicate unusual recumbency in view of the 

 fact that, in certain living sharks, the integuments are built up 

 about the bases of the spines, forming low, laterally compressed 

 prominences from, which their tips protrude, as is the case in Cestra- 

 cion. We hesitate, however, in recognizing this feature as possessing 

 generic importance in contradistinction to the narrow, more regu- 

 larly and gradually tapering spines with which the latter have been 

 generically associated. 



The specific relations of the present specimen with previously 

 described spines are perhaps most intimate with the Kinderhook 

 species, Ctenacanthus speciosus, St. J. and W., and C. spectabilis, 

 St. J. and W. Its distinguishing features, however, are so pro- 

 nounced as not to require detailed comparison. 



Geological position and locality: St. Louis limestone ; Alton, 111. 



