Permian of JVeiv Mexico. 387 



is peculiar. A broad flange is directed inward, nearly verti- 

 cally, opposite the middle of the articular surface, concave in 

 front. The suture separating the prearticular from the artic- 

 ular is very conspicuous, passing back over the flange. In 

 front the prearticular passes far forward, between the upper 

 opening to the cavity of mandible and the elongated foramen 

 near the middle of the inner side before the middle of the bone 

 antero-posteriorly. A fracture of the mandible a little in 

 front of the articular shows a large cavity with an elongated 

 opening above back of the teeth. The elongated vacuity is 

 bounded by the angular below, by the splenial in front, by the 

 prearticular above behind, anteriorly apparently by the coro- 

 noid. The splenial is very broadly visible on the under side 

 of the mandible, the suture between it and the dentary begin- 

 ning some distance in front of the posterior end of the median 

 symphysis, and extending back nearly as far as the posterior 

 end of the internal vacuity. On the left 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 and apparently partly covering the internal vacuity in 

 front. Interesting is the fact that the existeuce of a separate 

 prearticular is demonstrated beyond doubt in this specimen, 

 and also that the splenials meet in a median symphysis in front 

 as in Labidosaurus, and probably all the Cotylosauria. 



Vertebrae. — Eighteen presacral vertebrae have been cleared 

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

 lengths of these vertebras are almost exactly the same through- 

 out ; in front of them the vertebrae above the pectoral girdle 

 have not yet been exposed ; the space in which they lie corre- 

 sponds exactly with that of five vertebra? following them, and 

 that is doubtless the number hidden in the matrix. In front 

 of these the atlas and axis have been partially exposed, giving 

 twenty-five as the total number of presacral vertebras. The 

 first of the series exposed below, the eighteenth presacral, has 

 a shallow fossa or flattened surface on the under side in the 

 middle, which fossa increases in depth posteriorly, a very char- 

 acteristic feature which seems to separate this form from any- 

 thing hitherto described and especially Diasparactus Case. 

 The outline of the centra both on the sides and below, antero- 

 posteriorly, is deeply concave. The arch has a marked resem- 

 blance to that of Diadecles, so far as they have been worked 

 out, save that there is no trace of a hyposphene anywhere in 

 the series and the rib articulation is continuous from the arch 

 to the centrum, as in Labidosaurus. All the observed ribs 

 are single-headed, but expanded, that is without an emargina- 



