VEETEBKATES. 217 



tubes are raised into crater-like prominences encircled by delicate 

 radi, giving to the surface a beautiful stellate ornamentation very 

 like that so commonly met with in the teeth of Cochliodonts. The 

 general outline agrees with that of typical examples, the posterior 

 margin in the worn condition conforming to the bands of growth ; 

 but the coronal contour shows a greater fullness in the inner half, 

 in which particular it bears greater resemblance to the Chester 

 species Psammodus anf/idaris, N. and W. However, similarly pre- 

 served examples from Alton show a less regular concavity in the 

 transverse profile and corresponding fullness in the inner half, ap- 

 proximating the conditions observable in the Michigan example. It 

 is much to be desired that larger materials from the latter quarter 

 be obtained, in order to trace more definitely the characteristics of 

 its piscine fauna and its relations to that of the southern and south- 

 western areas of the same deposits. 



The maxillary to.oth bears somewhat marked resemblance to the 

 same form of teeth occurring in the Upper Burlington limestone, 

 Pxammodus Lovianus, but the associate mandibular teeth are quite 

 different from those identified with the latter species, as will be 

 manifest on comparison of the illustrations given of the two species. 

 We regret the lack of material permitting detail comparison with 

 the European species P. porosus, Agass., although little doubt is 

 entertained respecting the specific distinctness of the above described 

 forms. 



Geological position and localities: Not uncommon in the St. Louis 

 limestone ; St. Louis, Mo., Alton, and Monroe Co., 111., Grand Eapids, 

 Michigan. 



PSAMMODUS C^LATTJS, St. J. and W. 



PI. XVIII, Fig. 1. 



A fragment of a small tooth from the St. Louis formation at 

 Pella, offers so well-marked peculiarities that we have decided to 

 notice it under the above designation. It is homologous with the 

 form elsewhere referred to the upper jaw, and is distinguished from 

 Psammodus plenus, of the same formation, by the concavity in the 

 curvature of the outer border, the strongly produced postero-outer 

 angle, more strongly inbeveled anterior basal wall, and the rugation 

 of the coronal surface, which presents a very moderate longitudinal 

 convexity and transversely depressed contour. The length of the 

 tooth along the outer border between the acute posterior and the 



