44 REVISION OF THE PELYCOSAURIA. 



surfaces, indicating a terrestrial habit as distinguished from a probably aquatic one in 

 the former genus. The supracondylar foramen is present in this genus, and the prox- 

 imal articular surface winds obliqviely round the expanded extremity of the bone. 



"The separate jaws of D. incisiviis show well the character of the dental inser- 

 tions. A strong thickening of the inner wall of the maxillary bone is all that repre- 

 sents the palatine lamina. This enlargement does not extend to the level of the 

 external alveolar margin, which thus forms a parapet. The roots of the teeth are 

 long, and are contained in deep alveoli of the palatine thickening; but the portion of 

 them which projects beyond the alveoli is adherent to the external parapet by the 

 side, and hence the teeth appear to be pleurodont. They are shed in after the absorp- 

 tion of the root in consequence of the presence of the crown of the successional tooth. 

 The process commences at the inner alveolar border, and extends inwards and 

 upwards, invading the palatine wall of the maxillary- bone." 



In 1880 (48) Cope added materially to his description of the genus 

 Dimetrodon. 



" In both specimens of D. incisiviis^ portions of the palato-pterygoid arch are 

 attached to the maxillary bone. One of these elements is an oval plate with a thick- 

 ening of its inferior side, so as to bevel the long border farthest from the maxillary 

 bone. The surface thus produced is thickly studded with small conical teeth irregu- 

 larly disposed. 



"A second tooth-bearing element of the palate is adjacent to the last. It is a 

 massive plate, the ends of which are produced in opposite directions ; the one into a 

 massive shorter prominence ; the other longer and plate-like. Between these prolon- 

 gations, the inferior edge of the bone bears a single row of well-developed teeth. The 

 patch of small teeth first described, commences at the extremity from which the 

 longest process rises on the opposite side of the series of large teeth. This Z-shaped 

 bone is, from its massive character, generally preserved. * * * 



"The posterior part of the skull of one of the specimens above mentioned displays 

 typical reptilian characters. The occipital condyle is not perforated, nor divided by 

 sutures. The exoccipital bones project well backwards. The lateral walls of the 

 brain case are massive as far forward as the exit of the fifth pair of nerves ; anterior 

 to this point they were thin or wanting. The basisphenoid carries two parallel 

 descending laminae, which bound a deep median fissure, and then unite anteriorly. 

 Posteriorly they abut on a descending process, which is followed by a lid-like element 

 which is applied to a circular fossa with a raised border near the occipital condyle. 



"The articular face of the articular bone of the mandible consists of two parallel 

 cotyli, divided by a ridge of articular surface. This part of the jaw is nuicli depressed, 

 as in Eryops. The large teeth of the lower jaw are at the anterior extremity. 



"The neural spine of the axis is flat and elongate antero-posteriorly. From this 

 point the neural spines rise rapidly in elevation until on the dorsal region they are 

 many times as long as the diameters of the centra. The latter are not verj- unequal 

 in their proportions in different parts of the column. Those from the posterior 

 regions are less compressed than the dorsals and cervicals. The dorsals are separated 

 by intercentra below, which are small in the D. iiicisiviis, and larger in the D. gigas. 

 All the ribs are two-headed, commencing with the axis. All the cervical and dorsal 

 vertebrse have diapophyses with tubercular facets. The head of the rib is prolonged 

 downwards and forwards to the prominent border of the anterior articular face, against 

 which it abuts, but so far as yet observed without a corresponding facet. On the 



