VEKTEBRATES. 



face minutely and closely punctate. The proportionate dimensions 

 of large examples vary considerably, according to the state of trun- 

 cation of the inrolled extremity; in other respects, however, as in 

 general outline and coronal contour, the observable variation of in- 

 dividual teeth is comparatively unimportant, save what is attribu- 

 table to wear while in use. Both the anterior and posterior borders 

 are faintly undulated, suggesting their association with series of 

 smaller teeth. The latter condition is even more markedly exhibited 

 in the teeth (including this particular form,) from the Chester lime- 

 stone, Psephodus crenulatus, N. and W., with which the teeth de- 

 scribed under the name Aspidodus convolutus, N. and W., is specific- 

 ally identical. The posterior sinuations are fewer, indicating larger 

 teeth for the contiguous series than those at the opposite border. 

 The relations of crown to base are shown in the illustrations given 

 of the teeth in question. These teeth probably occupied a median 

 position upon the rami of the mandible. An ordinary-sized tooth 

 measures in greatest breadth between the inner angles 40 mm; 

 ditto, across outer extremity 25 m m ; length along antero-lateral 

 border 20 m m ; ditto, postero-lateral border 30 m m ; depth of tooth 

 at middle of anterior border 3mm; ditto, middle of inner margin 

 5mm; greatest length along coronal ridge 40 m m. The propor- 

 tions will, of course, be found to vary in individuals of the same 

 age, in accordance with the abbreviation of the outer extremity. 



Teeth strikingly in contrast with the preceding form, occurring in 

 the collection with the same frequency, but possessing superficial 

 characters which strongly suggest their intimate connection with that 

 form; may be regarded as the opposed form of the upper jaw. They 

 are distinguishable by their trapezoidal outline and relatively uniform 

 low coronal surface, and proportionally less robust build.- Outer ex- 

 tremity obliquely truncated in the same direction observed in the 

 previously mentioned form, or from the anterior angle outward and 

 downward, and forming nearly a right angle with the antero-lateral 

 border, the narrow coronal fold projecting beyond the furrowed base, 

 and somewhat irregular in outline, forming an articular edge with a 

 similar preceding tooth of the same series ; antero-lateral border 

 proportionately shorter than in the first noticed form, slightly 

 curved sigmoidally, nearly vertical, with a narrow channel defining 

 the crenulated coronal fold from the base, and more or less dis- 

 tinctly undulated; postero-lateral border somewhat more strongly 

 curved, obtusely rounded at the inner posterior angle, the crenulated 

 coronal fold slightly overarching the basal portion, which is also 



