82 BULLETIN UNITED STATES GEOLOGICAL SURVEY. [Vol.YI. 



M. 



_. , J, ^1.. T -n ^^ r. (anteroposterior 018 



Diameter of tnird maxillary at base ot crown < , . , ^ 



t transverse 015 



Length of seTenth maxillary from alveolus 026 



_,. ^ „ .-. J. ^,. n C anteroposterior 012 



Diameters oi crown at base oi cuttma: eases < , „„„ 



* ^ (transverse., 009 



The approximation to a two-rooted condition in some of tlie teeth is 

 a marked peculiarity of this species. The median groove is most ex- 

 tensive on the smaller maxillary tooth, extending into the base of the 

 crown. From observation on this species and on certain Dliiosanna, I 

 am under the impression that the two-rooted mammalian tooth is the 

 result of constant absorption of the median portion of the wide root of a 

 wide crown, due to the presence and pressure of the successional tooth. 



DI3IETR0D0N ^EUCIGER Cope, Paleoutological Bulletin i^o. o2, p. 7. 



This species was abundant during the Permian period. One of the 

 best preserved individuals in my collection is represented by the greater 

 X)art of the vertebral column and the pelvic and scaj)ular arches. 



The iDclvic arch resembles that of the D. (jigas in its general charac- 

 ters. For a tlescription of the latter see Paleoutological Bulletin IJfo. 32, 

 p. 6. The scapular arch also agrees with the descriptions I have given, 

 but possesses some parts which have been hitherto wanting. These are 

 the clavicles, as I suppose them to be. Each of these consists of a hori- 

 zontal portion and an ascending portion. The former is nearly a quar- 

 ter of a circle in form, with its convex outline j)resenting inwards and 

 backwards. The border is marked by radiating ridges which terminate 

 in a suture like rugosity of the edge. The lower surface curves rapidly 

 into the ascending portion, which is narrow and acuminate and about 

 twice as high as the horizontal portion extends inwards from its interior 

 border at the base. It displays on its external border an extensive 

 sutural surface for union with the scapula. The anterior extremity of 

 the epicoracoid lies on the superior (internal) face of the horizontal 

 portion of the clavicle, extending nearly to its anterior border. Its ex- 

 ternal margin thins out in contact with the clavicle. 



The above structure is peculiar among Eeptilia in the widely-expanded 

 subtriangular distal part of the clavicle. It presents no greater resem- 

 blance to any known mammal. Its character is more that of the Stego 

 cephalous BatracMa, and it is a good deal like the lateral thoracic shield 

 of the animals of that group, as, for instance, Actinodon of Gaudry and 

 Cricotus m. From the sutural character of the borders of this part I 

 anticipate the discovery of a median or mesosternal element. It is pos- 

 sible that the piece attached to its superior face, which I have above 

 alluded to as epicoracoid, may be a part of this bone disji)laced laterally. 

 The resemblance of the humerus of this and of allied genera to that 

 of the mole was reported by the writer in the American JSTaturalist, 1878, 

 p. 408 (June), and the nearer resemblance to Echidna was asserted in 

 the same journal, 1878, p. 830 (December). 



