50 ALUS. [Vol. XVIII. 



scribed and named by Sagemehl, is shown, more or less developed, 

 in his figures, and may be called the supratemporal groove. 



The general outer level of the dorsal surface of the posterior 

 part of the skull is given in the outer edges of the five ridges 

 that separate the six dorsal grooves one from another, and is 

 decidedly convex, the dorsal surface of the skull, on each side, 

 sloping laterally, backward and downward to its postero-lateral 

 corner. 



The Dilatator Groove {dgr) is shorter than the other two, 

 looks upward and decidedly laterally, and lies on the external sur- 

 faces of the frontal and squamosal bones and the postorbital ossi- 

 fication. Posteriorly it opens onto the lateral, and not onto the 

 posterior surface of the skull, the opening extending from the 

 dorso-lateral corner of the postorbital process backward to the en- 

 larged hind end of the ridge that separates the groove from the 

 temporal groove. The posterior half of this opening lies directly 

 dorsal to the two facets that receive the two dorsal articular heads 

 of the hyomandibular, the anterior facet often cutting slightly 

 into the dorso-lateral edge of the skull and hence into the edge 

 of the opening. Directly above the anterior edge of this anterior 

 facet, and slightly in front of the middle of the opening of the 

 groove, there is a small process, more or less marked in different 

 specimens. From this process a slight ridge runs forward and 

 mesially along the floor of the groove, the process and ridge 

 separating the opening and groove into tw^o somew^hat different 

 regions. The mesial and posterior of these two regions lies at 

 a slightly deeper level than the other one, the difference in level 

 being most marked toward the hind end of the groove. The 

 process and ridge mark the line of origin of a longitudinal tendon 

 of the dilatator operculi muscle. 



The Temporal Groove {tgr) is the middle one of the three, 

 and is deeper than either of the other two. It opens, at its hind 

 end, onto the posterior surface of the skull, the opening occupying 

 the entire space between the dorso-posterior corner of the exoc- 

 cipitale and the postero-dorso-lateral corner of the skull. The 

 bottom of the groove, at about the middle of its length, changes 

 abruptly in level, the posterior portion hing considerably deeper 

 than the anterior one. At the antcro-mesial corner of the pos- 



