— 168 — 



by the oticus lateralis and one by tlie supratemporalis lateralis vagi. Near the middle of the bone, 

 between the organs innervated by the oticus and supratemporalis lateralis vagi, the main infraorbital 

 canal anastomoses with the dorsal end of the preopercular canal. 



The primary component of the pterotic has a thickened anterior portion but is elsewhere 

 a thin plate of bone which lines the ventral surface of the angular dermal component of the bone. 

 It is traversed by the external semicircular canal, but otherwise has no bounding relations to the 

 cranial cavity. This is due to the presence of a large and deep depression on the lateral surface of 

 the brain case, this depression pinching the skull, so to speak, to such an extent that the primary 

 component of the pterotic has been pressed upward against the under surface of its dermal component, 

 has ceased to form part of the apparent lateral surface of the brain case, and simply lines the ventral 

 and ventro-mesial surfaces of the overhanging roof of the depression. This depression includes both 

 the proötic and subtemporal fossae of Scorpaena, and it has, in addition, a large posterior extension 

 which lies posterior to the arch of the external semicircular canal and is not found in Scorpaena. The 

 entire depression may, however, be called the subtemporal fossa, the fossae of opposite sides giving 

 to the brain case, in ventral views, an hour-glass-shaped appearance. The adductores hyomandi- 

 bularis and operculi have their origins in the fossa, arising mainly on a band of cartilage that separates 

 the pterotic from the exoccipital. The levators of the first four branchial arches have their origins 

 in the anterior portion of the fossa, anterior to the adductor hyomandibularis, while the fifth levator 

 muscle has its origin ventro-mesial to the adductor operculi, between that muscle and the foramina 

 for the vagus and glossopharyngeus. This fifth levator perforates, to reach its surface of origin, 

 a mass of tissue that looks like degenerate glandulär tissue and that must be the homologue of what 

 I considered as the thymus in Amia ('97, p. 643). Whatever it may be, this tissue fills the entire fossa 

 ventro-mesial and posterior to the adductores hyomandibularis and operculi, and the fossa would 

 seem to be developed in some relation to it rather than as any consequence of the origins of the several 

 muscles here. 



On the anterior edge of the thickened anterior portion of the primary component of the pterotic 

 there are two fossae, the larger, mesial one lying on the internal surface of the bone while the smaller, 

 lateral one lies on its external surface. The mesial fossa lodges the anterior portion of the external 

 semicircular canal, and is in commiinication with the dorso-antero-lateral end of the tall recess already 

 described on the internal surface of the proötic and sphenotic ; the posterior portion of the semicircular 

 canal traversing the thin posterior portion of the auto-pterotic, and forming a prominent ridge on 

 its external surface. The lateral one of the two fossae forms the posterior half of the dilatator fossa, 

 the anterior half of that fossa lying in the sphenotic. Immediately posterior to the dilatator fossa, 

 and so placed that it seems to form part of the mouth of that fossa, there is, on the external surface of the 

 auto-pterotic, a large round articular fossa which gives articulation to the regulär posterior articular 

 head of the hyomandibular. The bind edge of this articular fossa is thickened and grooved to form 

 an elongated, transversely placed and slightly convex articular surface, which gives a sliding articu- 

 lation to the posterior one of the four articular heads of the hyomandibular. Immediately dorso- 

 lateral to this articular surface is the Perforation by which the preopercular latero-sensory canal 

 joins the main infraorbital canal. 



The auto-pterotic is bounded anteriorly by the sphenotic and proötic, and also by a small 

 interspace of cartilage between the adjoining edges of those two bones. It is separated from the 

 exoccipital by a band of cartilage, and beneath (dorsal to) this band it is in contact with the lateral 



