88 FISHES. 



some anatomists.-^ The alisphenoids (11) (Prooticum) form 

 sutures posteriorly witli the basi- and ex-occipitals, and meet 

 each other in the median line at the bottom of the cerebral 

 cavity ; they contribute to the formation of a hollow in which 

 the hypophysis cerebri and the saccus vasculosus are received ; 

 in conjunction with the exoccipital it forms another hollow 

 for the reception of the vestibulum ; generally it is perforated 

 by the Trigeminal and Facial nerves. The paroccipitals (9) 

 {Epioticum) lodge a portion of the posterior vertical semi- 

 circular canal, and form a projection of the skull on each side 

 of the occipital crest, to which a terminal branch of the scapu- 

 lar arch is attached. The Mastoid (12 + 13) (OpistJioficum) 

 occupies the postero-external projection of the head ; it en- 

 closes a part of the external semicircular canal ; is generally 

 coalesced with a membrane-bone, the superficial squamosal, 

 which emits a process for the suspension of the scapular 

 arch, and is frequently, as in the Perch, divided into two 

 separate bones. 



The anterior portion of the skull varies greatly as regards 

 form, which is chiefly dependent on the extent of the cerebral 

 cavity ; if the latter is advanced far forwards, the lateral walls 

 of the primordial cranium are protected by more developed 

 ossifications than if the cerebral cavity is shortened by the 

 presence of a wide and deep orbit. In the latter case parts 

 which normally form the side of the skull are situated in 

 front of the brain-case, between it and the orbit, and gener- 

 ally reduced in extent, often replaced by membranes ; especi- 

 ally the interorbital septum may be reduced to membrane. 

 The most constant ossifications of this part of the skull are 

 the orhitosjohcnoids (14), which join the upper anterior margin 

 of the alisphenoids. They vary much with regard to their 

 development — they are small in Gadoids; larger in the 

 Perch, Pike, Salmonoids, Macrodon, and the Clupeoids ; and 



^ As first proposed by Huxley. 



