18 ILLINOIS BIOLOGICAL MONOGRAPHS [188 



Mesapterygoid. — The mesapterygoid, the ectopterygoid of some 

 authors, is a long, slender, curved bone which unites the palatine 

 with the quadrate. The anterior end of the mesapterygoid lies in the 

 groove on the dorsal edge of the palatine, from there it passes caudad 

 for a distance, then it curves ventrally and articulates with the anterior 

 edge of the quadrate. It is flattened laterally at the posterior and 

 middle parts but at the anterior end it is flattened dorso-ventrally. 

 There is a small groove on the ventral edge where it lies in a groove 

 on the dorsal edge of the palatine. On the dorsal edge of the mesaptery- 

 goid there is a long slender groove for a membranous bone, the en- 

 topterygoid, which forms a part of the ventral boundary of the orbit. 

 At the point where the bone curves ventrally there is a plate-like process 

 on the dorsal edge which connects with the mesapterygoid and the 

 entopterygoid. On the posterior edge the bone possesses a slight groove 

 where it unites with the quadrate. The bone tapers to a point at its 

 postero-ventral end. 



Hyomandibular. — The hyomandibular is shaped like an arrow 

 head with the shank extending from the ventral edge. The anterior 

 end is drawn out into a thin, pointed, plate-like process. Slightly dorso- 

 caudad to the pointed end is a thickened part which passes ventro- 

 caudad across the middle of the head and also across the shank, thus 

 forming an irregular cross with the shank. At the anterior end of the 

 bar forming the cross is a facet, the anterior head of which articulates 

 with the facet of the sphenotic directly above the prootic arcade. On 

 the posterior end of the cross bar there is a facet for the articulation 

 with the opercular. The shank passes upward through the body of the 

 bone and forms a double facet, the posterior head of which articulates 

 with the facet of the pterotic immediately posterior to the dilatator 

 fossa. The dorso-caudad part of the posterior head articulates with a 

 flange-like process dorso-caudad to the facet. The long shank extends 

 ventrally and is united to the symplectic at its ventral end by a mass 

 of cartilage. The metapterygoid articulates with the anterior process of 

 the hyomandibular which is a thin membrane bone filling the space 

 between the anterior bar of the cross and the shank. Likewise the spaces 

 between the other bars are also filled with a thin sheet of membrane 

 bone, thus leaving slight depressions between the bars. There is a deep 

 pit on the mesial surface at the angle between the anterior bar and the 

 shank, which gives rise to two canals. One passes ventrally into the 

 bone and opens on the lateral surface of the shank just anterior to the 

 ridge which bounds the groove for the articulation of the preopercular. 

 This canal bears the truncus hyoideo-mandibularis facialis vessel. The 

 other cajial passes ventrad and caudad, dividing into two parts; one 

 opens to the exterior at the dorsal edge of the groove for the articulation 



