A DESCRIPTION OF THE GENUS DIMETRODON, COPE. 39 



between prosquamosal and the quadrate. On the inferior surface a strong process runs 

 inwards and covers a part of the external edge of the quadrate. This process is notched 

 by the deep pit already described. Anteriorly the bone became very thin and underlay 

 the prosquamosal for quite a distance. 



The squamosal joins the quadrate as already described. Externally it joins the pro- 

 squamosals, the two bones meeting with everted edges to form a narrow ridge. The cranial 

 end of the squamosal was connected with the distal ends of the parietals by a narrow 

 squamous suture. 



The prosquamosal has been very largely denned in describing the other bones. The 

 superior surface is marked by a prominent ridge, which curves forwards and inwards until 

 it overhangs the ridge formed at the union of the squamosal and prosquamosal. The 

 anterior process became very slender and narrow and joined the posterior prolongation of 

 the postorbital, thus forming the upper temporal arch so characteristic for the Rhyncho- 

 cephalia and the whole group of Archosauria. 



The bones forming the cranium are all preserved, free from distortion and in their 

 natural position. The whole region resembles Sphenodon in many particulars, but the 

 obliteration of many of the sutures makes it impossible to compare exactly the separate 

 elements (PL I, Figs. 11-14). The cranium is formed by the union of the basioccipital, 

 exoccipitals, supraoccipital, the petrosals and the very large paroccipitals. The basisphe- 

 noid is completely free from the basioccipital. The connection was, without any doubt, 

 by cartilage. 



The basioccipital forms the lower half of the occipital condyle. The lower part of 

 the condylar portion is rounded, and the upper comes to a sharp point between the 

 exoccipitals. The point of union between the three bones is marked in some specimens 

 by a deep pit, the anterior prolongation of the chordal canal. The lower surface is con- 

 verted into a shallow groove by two descending flanges of bone. These meet laterally 

 two other flanges from the paroccipitals, and the suture line is marked by a sharp con- 

 striction. Anterior to the trough described the lower surface of the bone rises at an angle 

 of nearly ninety degrees. This face is excavated near its centre by a funnel-like depres- 

 sion, at the base of which lies the foramen for the Eustachian tube, as in the Crocodilia. 

 This region is greatly swollen and contains the petrosals, though the sutures are entirely 

 obliterated. 



The exoccipitals form the borders of the foramen magnum, the basioccipital being 

 excluded by their union below. The superior portions are very slender, and form only 

 a narrow surface around the foramen. Inferiorly they become larger and form the supe- 

 rior half of the occipital condyle. They are pierced near their posterior edge by the con- 

 dylar foramina. 



