VERTEBRATES. 127 



which are near the summit all smooth. The anterior medial 

 carina throughout the entire length of the spine, and toward 

 the base 3-4 of the anterior lateral carina ornamented with 

 numerous, obliquely transverse, prominent rings; those of the 

 lateral carinas passing into tubercles and becoming obsolete up- 

 ward. 



This is a remarkably slender, as well as beautiful, spine ; the ornamentation 

 being exceedingly neat and sharply defined. It is evidently closely allied to 

 C. distans, McCoy, from the Mountain limestone of Armagh, Ireland, which is 

 nearly as slender, and in its general aspect must be very similar. From the 

 description and figure of C. distans, given by Prof. McCoy, {Brit. Palaeozoic 

 Fossils, p. 625, pi. 3K,Jig. 15), I infer, however, that there are well marked 

 differences between the American and European fossil, which will readily serve 

 to distinguish them. In G. distans the form is somewhat more robust and 

 more curved, the teeth of the posterior margin less numerous and less depress- 

 ed, and the lateral longitudinal carinas, instead of being as in our fossil nearly 

 all smooth, are all crenulated by transverse rings or tubercles. 



From the other species of Ctenacanthtis, described by McCoy, Agassiz and 

 others, that before us is so widely separated that no comparison with them is 

 necessary. 



Formation and locality: St. Louis limestone, St. Louis, Missouri. 



