8 ILLINOIS BIOLOGICAL MONOGRAPHS [178 



ventrad along the lateral edge of the brain ease. The outer edge of this 

 flange narrows do^vn and, as it passes ventrad, it is drawn out to a thin 

 process; this flange and the thin process clsisp the small alisphenoid 

 between them as in Scorpaena described by Allis. The ventral surface 

 of the posterior portion is smooth and in connection with supra-occipital 

 which forms a plate beneath the parietals and posterior part of the 

 frontals forms the roof of the brain case. 



Post frontal. — The postfrontal (Fig. 1, pfo) is a small tube- 

 like bone similar to the one described by Allis in Scorpaena scorpus 

 except that it is without spines. It lies on the part of the dorsal surface 

 of the sphenotic which is not covered by the frontal and pterotic. The 

 tube extends from the lateral edge of the sphenotic caudo-mesad to the 

 anterior end of the temporal fossa. 



Sphenotic. — The sphenotics (Figs. 1, 6, spo) are located at the 

 lateral margins of the cranium, just posterior to the orbits and, in 

 conjunction with the lateral expansions of the frontals they form the 

 posterior boundary of the orbit. The dorsal surface of each is nearly 

 flat, except for a groove formed for the postfrontal bone which lies within 

 it. It thus forms supports for the frontal, the postfrontal, and the ptero- 

 tic. It forms the anterior, lateral, and ventral portions of the cranium. 

 The anterior margin borders on the orbit and the ventro-lateral edge 

 forms a facet for the articulation with the anterior head of the hyomand- 

 ibular. Directly dorso-anterior to the facet is a roughened surface which 

 serves for the attachment of one of the palatal muscles. Dorso-posterior 

 to the facet is a depression, the dilatator fossa, the posterior part of the 

 sphenotic forming the anterior part of the depression. Between the 

 dorsal part of the sphenotic and the frontal, beneath the postfrontal, 

 there is a small foramen which transmits one of the branches of the 

 otic vessels. The sphenotic also forms a part of the internal brain 

 case; the internal surface is smooth and has two depressions, separated 

 by a thin partition of bone which extends mesad into the brain cavity. 



Prootic. — The prootic (Fig. 6, pro) forms a large part of the 

 lateral surface of the cranium. It is bounded dorsally by the sphenotic 

 and pterotic, anteriorly by the alisphenoid and the dorso-lateral process 

 of the parasphenoid, ventrally by the parasphenoid, and posteriorly 

 by the basi-occipital and exoccipital. MesiaUy it articulates with the 

 basisphenoid and the prootic of the opposite side. On the ventral sur- 

 face it is overlapped by the lateral edges of the posterior portion of the 

 parasphenoid. The prootics form the lateral walls of the brain case 

 and also the roof and walls of the myodome. The brain cavity and the 

 myodome are separated by a mesial longitudinal partition-like process 

 which unites with a similar one from the opposite side in the median 

 line. This partition begins at the basioccipital, which also forms the 



