— 59 — 



of its dorsal surface gives support to the overlying parieto-extrascapular, the remaining part of that 

 surface forming the lateral portion of the floor of the supra temporal pocket; this latter part of the bone, 

 as alreadv explained, lying on and appearing as a part of the posterior surface of the skull. The 

 posterior surface of the bone forms part of the posterior surface of the skull; its lateral surface forming 

 the mesial wall of the temporal fossa. The angle between these two latter surfaces forms a strong, 

 epiotic ridge, which lies in a nearly vertical position near the middle of the corresponding half of 

 the posterior surface of the skull, forms the mesial boundary of the posterior opening of the tem- 

 poral fossa, and, with its ventral Prolongation on the posterior surface of the exoccipital, marks the 

 course of the posterior semicircular canal. The summit of the bone is directed dorso-postero-laterally, 

 and from it a flat, and often sharp and slender process arises, directed postero-laterally and slightly 

 ventrally. On the dorsal surface of this process, between it and the overhanging posterior portion 

 of the parieto-extrascapular, the epiotic process of the suprascapular rests, the two processes roofing 

 the hind end of the temporal fossa. The cerebral surface of the bone is wholly occupied by a deep 

 conical pit which forms part of the labyrinth recess and lodges the dorsal portion of the posterior 

 semicircular canal, that canal piercing the ventro-posterior wall of the recess and from there running 

 downward through the bone, internal to the epiotic ridge. 



The epiotic is bounded antero-laterally by the pterotic, ventrally by the exoccipital, and 

 mesially by the supraoccipital, with all of which bones it is in synchondrosis. 



SUPRAOCCIPITAL. 



The supraoccipital forms part of the dorsal and part of the posterior surface of the skull, 

 these two parts, or limbs of the bone lying at an obtuse angle to each other. From the postero-ventral 

 limb of the bone, and extending its füll length, a large thin spina occipitalis projects directly back- 

 ward, the ventral edge of the spina lying partly upon, and partly being enclosed between, the ad- 

 joining dorso-mesial edges of the medullary prolongations of the exoccipitals. The dorsal limb of 

 the bone projects forward between the lateral fontanelies in the roof of the primordial cranium, and 

 is overlapped anteriorly by the frontals, and laterally, on either side, by the parieto-extrascapular. 

 Between these three overlapping bones, a small and variable portion of the supraoccipital is exposed 

 on the dorsal surface of the skull and forms part of the floor of the subquadrangular groove on the 

 Vertex. A strong transverse ridge, projecting dorso-posteriorly, crosses the hind edge of the dorsal 

 surface of the bone, and against the anterior surface of this ridge the mesial processes of the parieto- 

 extrascapulars rest. The postero-ventral limb of the bone is crossed, near its dorsal end and on either 

 side of the spina occipitalis, by a more or less prominent transverse ridge, and that part of this limb 

 of the bone that lies dorsal to this ridge forms, on either side, the mesial portion of the supratemp- 

 oral pocket. The cerebral surface of the bone forms median portions of the roof and hind wall of 

 the cranial cavity. The bone is bounded ventrally by the exoccipitals, and laterally, on either side, 

 by the epiotic, with all of which bones it is in synchondrosis. Anteriorly the bone is edged with a band 

 of cartilage, anterior to which the lateral fontanelles of opposite sides are confluent; the band of 

 cartilage representing all that is found, in this fish, of the large postepiphysial cartilage of Scomber. 



CRANIAL CAVITY. 



The cranial cavity extends forward to about the middle of the orbit. The mid-longitudinal 

 line of the floor of the cavity slopes downward and backward from its anterior end to the anterior 



