\YILLISTON : A NEW SPECIES OF SAGENODUS. 179 



coming quite prominent at tlie beginning of the prolongation, when 

 it again decreases in height and disappears. The four sides of the 

 diamond are of nearly equal length, and the figure almost a square. 

 The anterior lateral borders show sutural surface, the bone project- 

 ing somewhat irregularly to underlap the pterygopalatines. The in- 

 ferior or buccal surface is gently convex transversely towards the 

 front, more nearly flat posteriorly. It is conspicuously marked by 

 two narrow ridges, beginning near the middle and running one to 

 each angle of the base of the posterior prolongation, and then disap- 

 jDearing in the thin upturned alate margin. The surface between 

 these ridges is concave and striate. Anteriorly the bone is marked by 

 an upraised surface in the form of a long triangle, with its apex at 

 the middle. The posterior process is thinned and narrowed poste- 

 riorly. 



The relations between the pterygopalatines and the basisphenoid 

 are shown in plate xxxvii, figure 2. One cannot be sure that the rela- 

 tive sizes of the two bones are correctly expressed, since the several 

 jaws in the collection vary somewhat in size. From the sutural sur- 

 face on the pterygoijalatines, however, I believe the proportions must 

 be about as drawn. 



The splenials with the dental plates are much narrower than are 

 the pterygopalatines and upper dental plates. The si^ecimen upon 

 which the following description is based is a practically complete 

 splenial and dental plate. The inferior border of the splenial is very 

 thin and shari3. The sutural surface for union with its mate is thick- 

 ened and flat; the lower half only is preserved in this specimen. 

 Back of the symphysis the inferior border is gently concave, when 

 seen from the side, to near the hind end, where it becomes convex, 

 ending in a ]3oint. The upper border, back of the dental jalate, is 

 also thin, and is curved sigmoidally to the tip. On the convexity of 

 the curve there is a slight roughening, as though for union with the 

 articular. The distal end is curved outward, as in Ceratodus, to in- 

 clude the articular cartilage. The inner surface below the dental 

 ridge is smooth throughout, gently concave, and standing nearly ver- 

 tically or slightly inclined outward back of the anterior i^ortion. In 

 front the surface turns inward to form a broad channel when in ap- 

 position with its mate. In general the shape of the splenial is much 

 like that of Geratodus, but is much more slender and elongated, the 

 sigmoid curve back of the demtal plate less pronounced, and the ex- 

 curved articular end more slender. 



The inner border of the dental plate corresponds to that of the 

 upiDer dental plates, though less strongly curved. This border forms 

 a strong ridge overhanging the inner surface. The general surface of 



