VERTEBRATES. 331 



which latter, though forming but a single row, are somewhat irregular 

 in their disposition or alternating from side to side, instead of forming 

 a direct line. Taking into view the posterior rami of this tooth, there 

 is no perceptible distinction between them and some of the irregularly 

 single-crested teeth noticed further on. The crown is worn, and although 

 the shape of the cusps is well preserved, there remains no trace of the 

 vertical costse or other ornamentation. 



Stejihatodtjs bicristattjs. 



PL 3, Fig. 32, 33, 35. 



Of the present form the collections contain several specimens, that 

 illustrated in fig. 32 being a very small individual but in a more perfect 

 state of preservation than its associates, which latter are much worn, 

 the basal region rarely indicating its true contour. In the perfect con- 

 dition, the teeth present a postero-anterior elongated outline, the base 

 slightly expanded laterally beyond the limits of the crown, and pro- 

 duced downward forming the rather strong lateral walls of the excava- 

 ted inferior surface, the anterior extremity considerably produced down- 

 ward in a laterally compressed wedge-shaped process, between which 

 and the tumid posterior extremity the outline is moderately arched 

 upward and rounded into the posterior coronal face ; a faint median 

 ridge extends from the anterior process into the inferior basal area, 

 which is also irregularly pitted. The coronal region is occupied by two 

 slightly sinuose rows of cusps, of which latter there are respectively 

 seven and eight in one or other row, depressed and quite regularly suc- 

 ceeding and increasing in size from front to rear, but without visible 

 ornamentation; the rows are separated by a deep, angular median 

 groove, and seem to originate in a single primary cusp at the anterior 

 extremity, the cusps of the respective rows are arranged in an irregular 

 alternating order. 



A variety apparently of the same form is represented in fig. 33, PI. 8, 

 in which the principal dental plate, though much worn, exhibits quite 

 the same characters, the only observable difference consisting in the less 

 distinct individualization of the rows of cusps, which are more crowded, 

 and towards the posterior extremity an extra denticle is developed in 

 one side, which was probably succeeded by others — the extreme posterior 

 portion of the tooth being broken off, so as not to show a third row of den- 

 ticles if such existed. But the most remarkable feature of the specimen 

 is the possession or association with a pair of very small, similarly 

 shaped dental plates, one on either side of the extreme anterior extrem- 

 ity, between which accessory teeth the principal plate seems to have 

 been wedged from below, but with which they are apparently firmly 



