YERTEBEATES. 203 



between which are most delicately wrought into short vertical ridges. 

 The anterior basal area presents a series of small protuberances corres- 

 ponding to the denticulations in the crest, as shown in fig. 20 b — a fea- 

 ture not observed in the other varieties. These teeth are also of smaller 

 size than the median teeth, from which fact, and their linear outline, 

 they may be regarded as having constituted the posterior rows upon 

 the jaws. 



For the means of studying this interesting form in all its phases, we 

 are indebted to the careful investigations of the local fauna of the Kiu- 

 derhook at Burlington, first begun by Mr. Frank Springer, and sub- 

 sequently in connection with Mr. Wachsmuth, which have been prose- 

 cuted with most valuable results in the acquisition of abundant and 

 authentic materials. The teeth under consideration differ in so marked 

 a degree from the upper Burlington form of the genus, as scarcely to 

 require detailed comparison. Among themselves, the varieties described 

 above exhibit every degree of gradation, by which they are shown to 

 be merely the varied dentition of one and the same form. While in the 

 ornamentation of the coronal region, as also the form of the base, there 

 is marked persistence in all the teeth, the coronal contour presents a 

 range of variableness which, in extreme examples, hardly affords a 

 clue to their specific identity, as is shown in what we have termed the 

 anterior and posterior teeth. 



Position and locality : Common in the lower fish-bed of the Kinder- 

 hook; a solitary specimen said to have been derived from the upper fish- 

 bed of the same formation, occurs in the collection of Mr. Giles, now 

 in the Museum of Comparative Zoology at Cambridge; Burlington, 

 Iowa. 



Mesodmodus explanatus, St. J. and W. 



PI. V, Fig. 15-17. 



Teeth minute. Base irregularly long-elliptical, or elliptical, in outline, 

 moderately arched upward, beveled in front from the narrow, generally 

 obscure shoulder, protuberauce proportionately large, globular, median, 

 inferior surface plane or faintly excavated, posterior face produced 

 downward and backward at a slight angle, or sometimes nearly at right 

 angles to the vertical plane of the crown, broadly rounded along the 

 posterior margin, extremities rounded, anterior face coarsely roughened, 

 posterior face more finely pitted. Crown prominent, compressed, slightly 

 constricted basally, lateral angles divergent, anterior face moderately 

 convex, posterior face less so, median prominence more or less produced, 

 median and obtusely rounded, or eccentric, the acute apex laterally 

 deflected towards the short extremity, recurved; lateral wings occupied 



