350 PALEONTOLOGY OF ILLINOIS. 



all, such as readily distinguish them as a whole from the allied th ugh 

 specifically distinct forms noticed from other horizons. Nor does this 

 variability shade off into these other specific forms, each of which, while 

 they present an equal degree of variation, are marked by certain persist- 

 ent features peculiar to them. 



From the St. Louis and Chester species, the present teeth may be 

 recognized by the proportionately greater size, narrower coronal belt, 

 and the peculiar compressed, carina-like crest, and the laterally com- 

 pressed median cone. As in those forms, the denticulated character of 

 the crest is quite variable, often almost obsolete, though, perhaps, never 

 entirely absent except when obliterated by attrition. 



Position and locality : Common in the ichthyic horizon of the upper 

 part of the Keokuk limestone ; vicinity of Warsaw, Illinois, Keokuk 

 and Bentonsport, Iowa, St. Francisville and Boouville, Missouri; appa- 

 rently rare at the latter locality, since few examples occur in the inter- 

 esting collection of Dr. Williams, made at that locality. 



Venustodus Leidyi, St. J. and W. 



PI. IX, Fig. 1-4. 

 Clwmatodus uenustus, Leidt, 1856. Trans. Am. Pail. Soc, Pbila., vol. xi, PI. v, fig. 19-21. 



The teeth which are arranged under the above specific designation 

 present considerable individual variation in form, but which may be 

 considered under two heads or varieties, as represented by individuals 

 from different parts of the jaws. 



First, teeth of small size, rectilinear in outline viewed from above, 

 gently arched between the extremities, a tooth of average size .50 inch, 

 in greatest length, and in breadth across the median portion of the 

 tooth .15 inch. The crown forms a rectilinear plate, very slightly dis- 

 turbed in the middle, lateral extremities angularly rounded, rising into 

 a subcorneal, more or less eccentric and usually truncated apex, which 

 forms an obtuse angle in the arched outline from which the crest grad- 

 ually declines to the extremities; the crest is more or less sharply 

 angular', and occupied by a variable number (10 to 18) of delicate 

 though well defined denticulations, which are generally more strongly 

 developed and numerous in one than in the other wing of the crest; the 

 concave face of the crown is gently depressed, about one third higher 

 than the opposite face, which latter is slightly arched vertically, and 

 more or less protuberant in the median region beneath the apex ; the 

 coronal folds form a continuous belt encircling the crown, broadest along 

 the concave margin, diminished to a narrow belt in the convex border, 

 and composed of a series of delicate interwoven thread-like imbrica- 

 tions, of which only the uppermost forms an uninterrupted line spanning 

 the crown ; both coronal surfaces are covered by a thin, smooth, enamel- 



