478 GEOLOGICAL SURVEY OF THE TERRITORIES. 



face. The dentition undergoes other still more important changes with 

 progressing age, so as to present the appearance of difference of species 

 at different periods. These are explained under the head of the P. Jiay- 

 denii^ the most abundantly represented in the collections. It may be 

 mentioned here that in neither P. haydenji nor P. turgidus is there any 

 ' eridence that more than two anterior molars are preceded, by decidu- 

 ous teeth, while the latter are present in many specimens. 



Pal^olagus agapetillus, Cope, Paleontological Bulletin, No. 15, p. 1. 

 Established on a mandibular ramus with the first and last permanent 

 molars just protruding. Size the least in the genus, not exceeding the 

 P. haydenii in the milk-dentition, but more robust and with larger in- 

 cisor-tooth. Form of the ramus wedge-shaped, contracted, and convex 

 on the outer side forward. Molars all composed of two columns, the 

 anterior the more elevated, the fore portion surrounded by its distinct en- 

 amel sheath, with a narrow intervening band of cementum. Posterior 

 molar much reduced in size ; posterior column of molars with a median 

 posterior rib, which forms a loop in section. Anterior column much 

 more elevated than posterior. The section of the slender incisor is 

 nearly a right-angled spherical triangle. 



Measurements. 



M, 



Length of molar series • 0.0100 



Length of penultimate molar 0020 



Depth at penultimate molar x. 0070 



Depth at first molar 0050 



Transverse thickness at first molar 0037 



Pal^olagtjs haydenii, Leidy. 



The earliest dentition of this species known to me is the presence of 

 the two deciduous molars, the first and second in position, before the 

 appearance of any of the permanent series. Each of these has two 

 roots, and the crown is composed of three lobes. In the first, the first 

 lobe is a simple cusp ; the two following are divided into two cusps 

 each 5 the second is similar, excepting that the simple cusp is at the pos- 

 terior end of the tooth. The grooves separating the lobes descend into 

 the alveolus on the outer side, but stop above it on the inner. The 

 measurements at this stage are — 



Measurements. 



M. 



Length of two milk-molars 0.0050 



Depth of ramus at No. 2 0042 



Depth of ramus at diastema , 0032 



In the next stage, the third permanent molar is projected, and has, 

 like the second deciduous, a posterior simple column, whose section 

 forms an odd cusp or lobe. The fourth true molar then follows, also with 

 an odd fifth lobe behind. This lobed form of the molars is so different 

 from that of the adult as to have Jed me to describe it as indicating 

 peculiar species under the name of Tricium avunculus and T. annm. 



In the next stage, the fifth small molar appears in view, and the second 

 permanent molar lifts its milk-jjredecessor oat of the way. In a very 

 short time, the posterior, or odd, columns entirely disappear, sinking into 

 the shaft, and the permanent molars assume the form characteristic of 

 the species. The last stage prior to maturity sees the first milk-molar 

 shed, and the younger portion of the first permanent molar protruded. 



