VERTEBRATES. 211 



direction, making an obtuse angle with the anterior margin and 

 which is strongly beveled from the acuminate culmination of the 

 outer coronal ridge; the crown is rather deeply depressed trans- 

 versely, and very moderately arched longitudinally, presenting a 

 beautifully sculptured surface which bears a striking resemblance to 

 the style of ornamentation observed in the typical specimen above 

 described. The specific identity of the present example with that 

 form, however, is merely conjectural. The specimen is of especial 

 interest from the fact that it belongs to an opposite row, all the 

 individuals of P. crassidus at present known being from the same 

 side. 



Geological position and localities: Upper Burlington limestone, main 

 fish-bed ; Quincy and Cedar Creek, Henderson county, 111. 



PSAMMODUS GRANDIS, St. J. and W. 



PI. XV, Fiff. 1-3. 



Teeth attain large size. Maxillary form subrhomboidal in out- 

 line, moderately arched longitudinally, gently depressed transversely. 

 Inner articular border nearly straight, one specimen showing a 

 slightly truncated posterior angle for the lesser articular facet, 

 while a smaller individual shows a faint angulation a little in 

 advance of the middle, but so disfigured by abrasion as not to 

 clearly show its character. Coronal enamel forming a narrow fold 

 more or less well defined from the deep basal portion ; outer border 

 not preserved ; anterior margin very gently curved or nearly straight ; 

 posterior margin gently arched, more or less irregularly undulated, 

 especially in mature teeth. Coronal surface in young examples 

 smooth and regularly convex and concave in the two diameters, 

 maturer individuals being traversed by more or less conspicuous 

 transverse undulations, and which are even faintly discernible in the 

 earlier-formed smaller teeth. The surface is elegantly sculptured, 

 the rugose lines showing a decided transverse arrangement, though 

 sometimes taking an oblique course over limited areas, and in the 

 more exposed parts presenting a reticulate appearance like Arabesque 

 designs, and in worn surfaces becoming obsolete where the ordinary 

 minute punctae appear. A small tooth presents the following rela- 

 tive proportions : Breadth across posterior margin, about 30 milli- 

 metres ; breadth across anterior margin 24 mm ; length along inner 

 articular border about 20 mm. 



