50 REVISION OF THE PELYCOSAURIA. 



Mcasinemenls. 



m. ^n. 



{Antero-posterior 0.031 Expanse of posterior zygapophyses 0.030 

 Transverse 034 Length of base of neural spine 025 

 Vertical 026 



Tliis is a synonym of Dinietrodon incisivus. 



Dimetrodon semiradlcatus Cope. 



Cope, Bull. U. S. Geol. and Geog. Survey Terr., vol. vi. Article 11, t88i, p. 80. 



Type : An imperfect skull with the bones beautifully preserved and naturally 

 cleaned. Anterior end of skull and right maxillary, orbital region, fragment of 

 pelvis, femur, and fragments. No. 4001 Am. Mus. Nat. Hist., Cope, coll. ; Big 

 Wichita river, Texas. 



Original description : "A considerable part of the skull and some limb bones 

 represent this species. There are no vertebrae referable to the specimen, but the two 

 maxillary and premaxillary bones support nearly all the teeth in an excellent state of 

 preservation. Continuity of the dental series is preserved by one maxillary bone or 

 the other, excepting just at the extremity, where there is a slight interruption on both 

 sides. On one of them it must be very slight. 



" There are three teeth in each premaxillary bone. In the maxillaiy I count 

 seventeen, with the bare possibility of the necessity of adding one more. The first 

 premaxillary and third maxillary teeth are of nearly equal size and are nuich larger 

 than the others, the second premaxillary only approaching them. The section of the 

 base of the first premaxillary is subtrifoliate, there being one groove on the inner, and 

 two on the external face. The section of the middle of the crown is more than a 

 semicircle, with the base convex. The two angles are the sections of two ridges, 

 which are both presented posteriorly, the one on the inner, the other on the external 

 face of the crown. The crown of the second premaxillary has the same form, but the 

 base has only slight traces of the grooves. The third premaxillary is a diminutive 

 of the second. 



" The crowns of the maxillary teeth differ from those of the premaxillaries in the 

 opposition of the cutting edges, which present anteriorly and posteriorly. The exter- 

 nal face is more convex than the internal. The crown of the large third tooth is not 

 expanded above the root but its antero-posterior diameter contracts regularly to the 

 apex. The crowns of the other teeth are wider at the base antero-posteriorly than the 

 root. They are slightly curved backwards, and their edges are more or less regularly 

 crenate. 



" Several peculiarities distinguish this species from the D. incisivus, with which 

 it agrees in size. In the first place, the section of the root, at and below the base of 

 the crown of the third or large maxillary tooth and of the seventh tooth posterior to 

 it, is of the form of a figure x directed antero-posteriorly. This is due to the deep 

 grooving of the tooth on the opposite sides at this point; the grooves not extending 

 on the crown. The grooves are deeper on the smaller teeth, giving it an almost 

 biradiculate character. In D. incisivus the sections of these teeth are subquadi-ate. 



" In the second place, the section of the base of the first incisor differs from that 

 of D. incisivus, where it is subquadrate with two opposite shallow grooves. Next, 

 the nostril excavates the border of the maxillary bone ; in D. incisivus, the nostril is 

 separated from that bone by the intervention of the nasal. In that species there are 

 but two premaxillary teeth; in D. semiradicahis there are three. 1 * * =(= 



