16 THE HISTORY OP THE PELYCOSAURIA, WITH 



all parts of the skeleton, including jaws of both sides with teeth, numerous vertebrae and 

 bones of the limbs. 



The neural arches are coossified with the centrum. The jaws are long and slender 

 and the teeth are of equal size. The number of teeth in the dentary is about twenty- 

 one. The mandibular articular face consists of two open parallel grooves, one shorter 

 than the other, extending obliquely to the long axis of the jaw. A dentigerous bone of 

 the palate is described. 



The vertebrae have simple elongated neural spines. Intercentra are indicated by 

 the shape of the vertebras, but not preserved. Traces of sutural articulation with the 

 neural arches remain. Many of the centra are much compressed and have a narrow, 

 sharp median keel. 



Dimetrodon is more fully described ; the vertebrae and a portion of the muzzle are 

 figured. Parts of the palatopterygoid are described, probably pieces of the palate and 

 pterygoid ; both bear teeth. The posterior part of the skull displays typical reptilian 

 characters. The occipital condyle is described as not perforated nor divided by sutures. 

 The exoccipital bones [paroccipital processes] project well backwards. The lateral walls 

 of the brain-case are massive as far forwards as the exit of the fifth pair of nerves ; ante- 

 rior 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. Pos- 

 teriorly 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 is the quadrate.] This part of the 

 jaw is much depressed, as in My ops. The large teeth of the lower jaw are at the ante- 

 rior 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. Intercentra are present in the 

 dorsals ; and all the ribs are two-headed, from the axis. All the cervical and dorsal ver- 

 tebrae have diapophyses with tubercular facets. The head of the rib is prolonged down- 

 wards 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. [The facet is on the 

 intercentrum.] On the caudal vertebrae the two facets of the ribs are approximated and 

 finally are not distinguished. They are here coossified with the centra. Then follow 

 short notes on humerus, pelvis and femur. 



Three species are distinguished as follows : 



Vertebral centra much compressed, acute below ; neural spines without processes D. incisivus. 



Vertebral centra less compressed, obtuse below ; neural spines without processes, larger D. gigas. 



Vertebral centra compressed, not acute below ; neural spines with cross projections D. cruciger. 



