VEBTEBBATES. 351 



like layer, usually worn away for a short distance below the crest on the 

 convex side, exposing to view a minute punctate structure. The base 

 forms a very low, relatively narrow plate, inferior surface slightly 

 oblique to the horizontal plane of the crown, with more or less angular 

 margins in front and behind, and in lateral extent nearly equal to that 

 of the crown. 



The second variety is represented by smaller teeth, which are princi- 

 pally distinguishable from those above described by their strong bow- 

 shaped or vertically arched outline and acuminate apex, but which 

 show a gradation from the acutely angular crest into the less strongly 

 apicnlate teeth. In the more prominently arched examples, a sharp, 

 angular ridge descends from the tip of the apex to the base of the 

 crown in the convex side, which latter is almost imperceptibly arched 

 vertically, the opposite face gently channeled in the long diameter; 

 towards the extremities, which are more or less asymmetrical, the crest 

 is delicately denticulated, the same as observed in the larger teeth. In 

 the character of the coronal folds, punctation of the convex margin of 

 the crest, form and position of the base, they are in no particular essen- 

 tially dissimilar to the large teeth. The relative position occupied by 

 these small teeth was probably anterior to those first noticed above — 

 the extremely arched forms may have occupied the symphysis of the 

 jaws corresponding to 'the prehensile teeth of the Cestracionts. In size 

 they also exhibit considerable variability, however, apparently never 

 attaining the dimensions of the posterior or median teeth. 



We are indebted to Mr. Yan Hornb for the use of a fine series illus- 

 trative of the varied phases presented by the interesting form under 

 consideration. The species was first made known by Dr. Leldt, who 

 described it under the name Chomatodus venustus. The original speci- 

 men, which was kindly loaned us for comparison, was derived from the 

 upper portion of the St. Louis limestone, probably from St. Louis, Mo., 

 and from Warsaw, 111., as we have most conclusively determined from 

 the nature of the matrix as well as the fossil itself, in both which 

 respects it agrees with the St. Louis specimens, and presents the same 

 strong contrast to the Keokuk species from the latter locality. Having 

 adopted the original specific application to designate the generic group, 

 of which the present form is regarded a typical representative, it gives 

 us pleasure to rededicate the species in honor of its discoverer. 



The above form is closely allied to V. arguttis, from which it is par- 

 ticularly distinguishable by the more rectilinear outline and the inter- 

 woven or reticulated character of the coronal folds, and which are 

 discernible in almost every condition in which the individuals are met 

 with. With the Keokuk and Upper Burlington forms it scarcely needs 

 comparison. 



