178 PALEONTOLOGY OF ILLINOIS. 



XYSTRODUS SIMPLEX, St. J. and W. 



PL VIII, Fig. 4, 5. 



Teeth small, trigonal in outline, rather strongly arched from within 

 outward. Antero-lateral border very slightly curved, presenting an 

 abrupt face from the sharp angulation above, and enveloped in a 

 rather deep fold of the coronal enamel which is more or less dis- 

 tinctly denned from the deep basal rim; inner margin forming 

 nearly a right angle with the anterior border, gently signioidal in 

 curvature with a shallow concavity in the region of the coronal 

 depression ; postero -lateral border converging toward point of inroll- 

 rnent at an angle of 25 to 30 with the opposite border, coronal 

 enamel forming a narrow rounded fold inbeveled to the shallow 

 basal rim. Coronal prominence slightly convex transversely, bordered 

 by the abrupt angulation along the antero-lateral side, rather steeply 

 sloped posteriorly where it is more or less distinctly defined by a 

 slight angulation, or merges into the broad posterior depression, 

 which is bordered by the steeply upraised alation. Surface showing 

 ordinary punctate structure over the worn areas of the crown, but 

 revealing no distinct arrangement in transverse parallel rows, in 

 which respect these teeth depart most widely from the typical species 

 of the genus. Greatest breadth across the inner margin, 7.5 mm.; 

 length along antero-lateral border, 8. 5 mm. 



Mr. Springer's collection contains a single specimen of a long nar- 

 row tooth which is supposed to represent the posterior form pertain- 

 ing to the upper jaw. In general coronal contour it is very like the 

 above described form of the mandible, showing the same abrupt 

 antero-lateral border, with, however, a relatively narrow enamel 

 fold, less steeply upraised alation and more gradually converging 

 postero-lateral border, which makes an angle of 20 with the oppo- 

 site border. The coronal punctae show faint traces of transverse 

 disposition, in which particulars it approaches the typical species of 

 Xystrodus. Length of tooth along antero-lateral border about 11.5 

 mm. ; breadth at inner margin about 7 mm. . 



The forms above described, which were doubtless associated upon 

 the jaws of the same species, while they hold most intimately to the 

 outline and coronal contour ascribable to Xystrodus, as illustrated 

 both by the type X. striatus, Agass., of the Mountain limestone of 

 Ireland, and the equally typical forms from the Lower Carboniferous 

 of America, offer in contrast the absence of distinct transverse 

 arrangement of the superficial puncta?. But, as has been mentioned, 



