FOSSIL AND RECENT 



131 



The basisphenoid is ' Y '-shaped, each wing contacting the pterosphenoid and 

 prootic in such a way as to exclude the latter from the optic foramen. The stem of 

 the basisphenoid is expanded distally. 



Throughout most of its length the parasphenoid is narrow, expanding only at the 

 level of the short ascending wings. Both anteriorly and posteriorly the parasphenoid 

 is marked by a median groove. Towards the rear of the orbit the parasphenoid 

 bears approximately 30 teeth. Each tooth is conical with a rounded tip. The 

 efferent pseudobranchial and internal carotid arteries pierce the parasphenoid at the 

 level of the ascending wings. 



Hyopalatine bones. The hyopalatine series of Pterothrissus is of a pattern charac- 

 teristic of other pterothrissids and albulids but rather different from that in the 

 primitive species of Osmeroides (0. lewesiensis and 0. levis), which resemble the 

 Elopoidei in this respect. 



The hyopalatine series of Pterothrissus is long and shallow. The hyomandibular 

 has a single broad articulatory head. The bone is thin in this region and this is 

 reflected in the narrow hyomandibular facet on the neurocranium. A stout ridge 

 runs the length of the hyomandibular shaft. Dorsally the posterior divisions of the 

 levator arcus palatini muscle insert on this ridge, while ventrally the ridge serves 

 as a site of origin for the A 2 division of the adductor mandibulae. Beneath the 

 hyomandibular head the posterior margin is expanded to form a rounded opercular 

 process. Upon the medial face a ridge is seen to grade into the opercular process ; 

 to this ridge are attached the deeper fibres of the levator opercularis. The hyo- 

 mandibular trunk of VII enters the hyomandibular on the medial surface near to the 

 anterior end of the head. Within the bone the nerve splits to two subequal branches, 

 both of which emerge on the posterior edge of the shaft. The upper, smaller, oper- 

 cular branch leaves the bone immediately beneath the opercular process while the 



enp 



mpt 



hm 



Ihm 



Fig. 64. Pterothrissus gissu Hilgendorf. Hyopalatine bones of left side in lateral view. 



Cartilage hatched. 



