32 AMERICAN PERMIAN VERTEBRATES 



back nearly as far as the posterior end of the internal vacuity. 

 On one side a piece about two inches in extent of this bone has 

 been peeled off from the dentary, showing the bone to be thin, 

 not more than six or eight millimeters in thickness. In front, 

 the splenial turns upward to cover the inner side of the mandible 

 below the teeth, covering the groove for Meckel's cartilage ante- 

 riorly. Interesting is the fact that the existence of a separate 

 prearticular bone is demonstrated beyond doubt in this genus, 

 and also that the splenials meet in a median anterior symphysis 

 as in Labidosaurus, the early long- snouted crocodiles, Plesiosauria, 

 etc. I give herewith a figure of the inner side of the mandible of 

 Labidosaurus, showing a very similar arrangement of all the bones ; 

 the suture separating the prearticular from the articular is very 

 evident in part, if not all its course. 



The separate prearticular bone is evidently characteristic 

 of the Cotylosauria if not all the early reptiles and amphibians. 

 I have, I think, demonstrated its existence in the Plesiosauria, 

 though Andrews, in his latest paper on the plesiosaurian mandible, 1 

 has not distinguished the suture separating it from the splenial. 

 The prearticular, with the same relations as those of the plesio- 

 saurs and cotylosaurs, is also present in the ichthyosaurs, though 

 called the coronoid by Merriam, Gilmore, and Andrews. In all 

 cases it passes below the large posterior opening of the mandible 

 for the entrance of the nerves, the vacuities below and in front of 

 the bone being for the most part merely openings into Meckel's 

 groove, as in the crocodiles, Erpetosuchus kansensis Moodie, etc. 



Vertebrae. Eighteen presacral vertebrae have been cleared 

 of the matrix, in a continuous series curved to the left. The 

 lengths of these vertebrae are almost exactly the same throughout; 

 in front of the exposed ones the vertebrae above the pectoral 

 girdle have not yet been cleared of their matrix; the space in which 

 they lie corresponds almost exactly to that of the five vertebrae 

 following them, and that is probably the number yet hidden. 

 In front of these the atlas and axis have been partly exposed, 

 giving twenty-five as the total number of presacral vertebrae, two 

 more than is known to exist in Seymouria, and probably two more 



1 Geological Magazine, VIII, 162, 1911. 



