202 



PALEONTOLOGY OF ILLINOIS. 



PSAMMODUS SPEINGERI, St. J. and W. 



PL XX, Fie. 411. 



Mandibular teeth rhomboidal in 

 outline, variable in the propor- 

 tions of length and breadth; an- 

 terior and posterior margins nearly 

 parallel; very gently and irregu- 

 larly curved forward, the coronal 

 belt occupying less than one-third 

 the vertical height of the tooth, 

 extending deepest in the anterior 

 margin, and in well preserved ex- 

 amples crenulated with vertical 

 plicae, producing an exceedingly 

 beautiful sculptured belt well de- 

 nned from the deep basal portion 

 by a slight inbeveled band; the 

 basal portion is also marked by 

 ^Srf ofmandibularteethofP>sammodMS irregular vertical rugosities, worn 

 ur F at!n|'sSac d e ibular series from above - trit ' surfaces exposing the coarse osse- 

 serifs Trails rse profile of mandibuiar OU8 struc ture which extends to the 

 serifs. c ' LonKitudinalproflleofmandibular dense, thin inferior layer. The 

 inner border is usually nearly straight and vertical, the enamel belt 

 continuous with that of the margins, though less distinctly sculptured ; 

 one or other angle of the border is truncated at a slight angle, 

 presenting an articular facet of variable extent, though not always 

 well denned from the greater articular face. Outer border presenting 

 a more or less rounded angle in front, gently curved and terminating 

 in a more or less produced angle or spur posteriorly; the coronal 

 enamel is well denned and margined posteriorly by a widish expan- 

 sion of the basal border, which is continued into the posterior spur. 

 Coronal surface very gently and regularly arched longitudinally, the 

 angles of the posterior and anterior margins and inner articular 

 border sharply defined, that along the outer border rounded from the 

 lesser coronal fold of that side, which is sometimes defined by a nar- 

 row impressed line, though usually merging into the shallow median 

 depression, from which rises the wider, very slight convexity that 

 occupies the inner portion of the surface. In some specimens the 

 coronal surface shows distinctly the prismatic structure, the minute 

 punctae being less discernible. 



