6 ILLINOIS BIOLOGICAL MONOGRAPHS [176 



frontal to the caudad end of the skiill. In the anterior half it is compar- 

 atively shallow, but throughout the posterior half it has a depth of 

 nearly one-third that of the skull. The fossa harbors some of the 

 muscles of the shoulder girdle. The dilatator fossae (Fig. 5-6) are 

 pits on the dorso-lateral edges of the skull, directly above the two articu- 

 lar facets of the hyomandibular. Anteriorly, each pit lies in the 

 sphenotic and posteriorly in the pterotic. The pterotic forms the greater 

 part of the roof of the pit while the most dorsal edge of the prootic forms 

 a part of the ventral edge. Two foramina are present in the pit for the 

 branches of the oticus vessels. The arch of the prootic Avhich forms 

 the bridge over the foramina for the fifth and seventh nerves, the 

 juglar vein and the external carotids is conspicuous in Opkidon. It 

 begins at the anterior central portion of the bone and extends dorsad to 

 the ventral edge of the anterior articular facet of the hyomandibular 

 on the sphenotic. 



Vomer. — The vomer (Fig. 1) is the anterior bone of the skull 

 and caps the cartilage from the ends of the ethmoid and the prefrontals. 

 It possesses both dorsal and ventral processes. The dorsal and anterior 

 one passes upward and posteriorly and articulates with the ethmoid and 

 prefrontals. The ventral process forms a part of the ventral surface; 

 it passes posteriorly and gradually tapers to a point in the parasphenoid. 

 A V-shaped slit is present in the dorsal process and forms a trough-like 

 cavity ^^^th the ventral process, alloAving the cartilaginous portion of 

 the ethmoid to pass into it. The anterior edge is broad V-shaped in 

 the median line and rounded from the median portion laterad forming a 

 large expansion at each side. Two kinds of teeth are present on the 

 vomer; an inner row of large canine-shaped ones and ail outer row of 

 the villiform variety in which the teeth are irregularly arranged. Both 

 kinds are slightly curved caudad and are placed in sockets. The 

 maxilla and premaxillary both articulate just above the vomer and cover 

 it entirely. 



Ethmoid. — The anterior dorsal portion of the ethmoid (Fig. 1, 

 6, eth) is strongly keeled, while the posterior portion is concave longi- 

 tudinally and ends between the frontals where it tapers to a point. 

 There is a narrow median ridge on the keeled portion extending its full 

 length Avith a V-shaped slit in the anterior end. The ventro-lateral 

 depression of the anterior portion forms a cavity between it and the 

 prefrontal, these parts forming the ventral and mesial osseous walls 

 of the nasal pit. The longitudinal groove in the concave portion is 

 formed by a dorso-lateral expansion on either side at its anterior end. 

 From this point the groove passes caudad and continues as the rectangu- 

 lar groove between the ridges of the frontals. The sides of the posterior 

 portion are eoneave from the point at which the lateral expansions ex- 



