a description of the genus dimetrodon, cope. 35 



Description of the Skeleton of Dimetrodon. 



The general shape of the skull can best be seen from the figures. There are two 

 temporal arches ; an upper, postorbito-squamosal arch, and a lower quadratojugal arch. 

 There is, of course, also a parietoquadrate arch. 



Seen from above the following openings are found in the skull : The anterior nares, 

 far in front, bounded by the premaxillaries, maxillaries and nasals ; the orbits, placed 

 nearly vertically, and surrounded by the frontals, prefrontals, lachrymals, jugals, post- 

 orbitals and postfrontals ; the supratemporal fossce, bounded by the parietal, postorbital, 

 prosquamosal and squamosal ; the infratemporal fossce, formed by the postorbital, jugal, 

 quadratojugal and prosquamosal. The pineal foramen is placed between the parietals. 

 The posttemporal fossce are surrounded by the posterior parietal processes, the squamosal, 

 the paroccipital processes and the supraoccipital. 



The premaxillaries (PI. I, Fig. 1) are small, strong, paired elements. They are 

 suturally united in the middle line, sending a slender process between the nasals. Behind 

 they are united with the maxillaries and at the union a deep notch is present. Their 

 anterior edge is rounded and carried upon a slender recurved process which borders the 

 nasal opening anteriorly and joins the nasals above. The lower portion of the opening- 

 is formed by the posterior part of the upper edge of the premaxillary, which is excavated 

 at the base of the superior process. Internally the two bones unite at their anterior 

 edges and show faces for the anterior ends of the vomers. There are three teeth in each 

 premaxillary ; the anterior large and strong, followed by two smaller ones. The nasals 

 (PL I, Fig. 2n) are long and slender bones, connected with the premaxillaries, maxil- 

 laries, prefrontals and frontals. They are suturally united in the middle line, and 

 diverging behind to enclose the pointed anterior ends of the frontals, which they overlie. 

 Anteriorly they receive the posterior prolongations of the premaxillaries between them. 

 Below they are united with the maxillaries and behind with the prefrontals. The 

 frontals (PI. I, Fig. 4) are paired. They are of peculiar shape. They form a very short 

 suture with the parietals, reach in front between the posterior ends of the nasals and send 

 out laterally slender processes which take part in the upper border of the orbit. The 

 frontals are flat and narrow, showing that the skull was not very broad. They join the 

 nasals, prefrontals, postfrontals and parietals. The parietals (PL I, Fig. 5) are very 

 small and short. They are not suturally united with the supraoccipital, but by cartilage. 

 Their posterior processes, which are first horizontal, but vertical at the distal end, join 

 the squamosals. The parietals are connected with the frontals, postfrontals, postorbitals, 

 squamosals, supraoccipital and paroccipital. The prefrontals (PL I, Fig. 3) are well 

 developed ; they take part in the anterior and upper border of the orbit. They join the 



