AiAini.AM) fiKor.ocK \i. St i!\ i;y 99 



M ^ i.ioi; \iis coi'i; ANTS Clark. 

 Il.ilr XII, I'i.us. 1, 'I; IM.ilc Xlll. Ki-s. '.^n. "il), C, Tm, 'll.. 



.yi/liiiliutlx ropctiiiiis i'\iivk, ls<.ir>. .Inliiis llupkins Univ. ('ire, vol. xv, ji. 4. 



J/ l/liobn I is coptim IIS VAiirk, IHiKi. Hull. Ill, U. S. Geol. Survey, p. 01, pi. vii, Ulth. 



DescripHdn. — Dentition lariic ami massive, the eoronal eiiiitimr 

 strongly ciii-\(mI antero-postei'iDrly, and iiKHlciately nrclied i'ritm side to 

 .side in the ii|i|ie!' jaw. hnt only \rv\ sli;4lilly so in the lower, 'i'l'ans- 

 vei'se sutni'i's stnni^ly ami regularly concave towacii the fruiit in ii|i|)i'i- 

 median tet'tli. less so in those of lower jaw, where they are sometimes 

 slightly serrated. lJi)i)er median teeth relatively longer than the lower, 

 l)nt often variahle in this respect; as a rule they are about five times as 

 hi'oad as they are long in the adult. Lateral teeth long and narrow, 

 arranged parallel with tlie main axis. Surface smootli ami |)olished 

 when unahraded, but becoming longitudinally striated and lissnred 

 when worn. Oral surface of lower dental plate exhibiting a slight 

 longitudinal depression along the central portion, and under surface cor- 

 respondingly thickened, the latter having an arcuate contour from side 

 to side. 



This species is very abundant in tlio Eocene of Maryland and Vir- 

 ginia, but is apparently unknown beyond tlu' limits of these states. A 

 large series of specimens has licen collected, tlius facilitating a compari- 

 son of successive growth stages, and of variations due to wear and other 

 causes. It is a remarkably massive form, the thickness of the median 

 teeth equalling that of 21. pacln/odon from the Miocene, and the trans- 

 verse section being very much the same. In the latter species, how- 

 ever, the transverse sutures of the median teeth are nearly straight, 

 being only slightly curved backward at the extremities, whereas the 

 present form is distinguished by the median sutures being uniformly 

 concave toward the front. The curvature of the suture-lines is usually 

 more pronounced in the upper dentition than in the lower, and in the 

 newest-formed teeth than in those more anteriorly situated. The me- 

 dian teeth of the lower dental plate are also somewhat shorter, as a rule, 

 than those in the upper jaw, and the coronal contour is much less arched 

 from side to side. 



