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profundi and also the encephalic branck of the jugular vein. The antero-ventral edge of the trigemino- 

 facialis Chamber is either notched, or perforated by a foramen which transmits the palatinus facialis 

 from the Chamber into the myodome, this nerve here, as in Trigla, issuing from the cranial cavity 

 through the facialis foramen and then running forward along the floor of the trigemino-facialis Chamber. 

 The edge of the orbital portion of the bone, dorsal to its mesial process, is notched to transmit the 

 oculomotorius, and dorsal to this notch, near the ventral edge of the alisphenoid, there is a second 

 but shallower notch for the nervus trochlearis. 



The MYODOME has proötic and basioccipital portions, and, excepting in that the basi- 

 sphenoid bone and the prepituitarv portions of the mesial processes of the proötics are replaced by 

 membrane, the canal is the exact equivalent of the canals of Trigla and Scorpaena. There being 

 no basispkenoid bone, the myodome, in the prepared skull, opens into the hind end of the orbit by 

 a wide median opening, bounded, on either side, by the ascending process of the parasphenoid. In 

 the middle line of the floor of the opening there is the median tooth-like process of tke paraspkenoid. 

 Tke kypopkysial fenestra extends nearly tke füll lengtk of tke myodome, but is muck narrower in 

 tke basioccipital region tkan in tke proötic. Tke fenestra is completely closed by tke underlying 

 paraspkenoid, tke myodome not opening, posteriorly, on tke ventral surface of tke skull. Tke roof 

 of tke basioccipital portion of tke canal is formed by a tkin plate of bone, wkick separates tkis part 

 of tke myodome from tke overlying cavum sinus imparis. Tke cavum sinus imparis extends poste- 

 riorly sligktly furtker than the myodome, the pointed ends of both canals being directed toward 

 the point of the conical vertebra-like depression on the hind end of the basioccipital. 



The BASIOCCIPITAL is normal, but presents, in median-vertical section, a marked feature. 

 The shallow conical vertebra-like depression in tke kind end of tke bone, in suck sections, is lined 

 by a superficial layer of dense bone differing markedly in appearance from tke deeper portions of 

 tke bone. A tkinner layer of similar bone lines tke deeper conical depression, in tke anterior end 

 of tke bone, tkat forms tke kind end of tke myodome. Tke ends of tkese two cones approack eack 

 otker, and tke dense bone lining tbem is continued, in tke middle line of tke bone, from one cone 

 to tke otker. In Scomber I described ('03, p. 102) a similar but muck less pronounced line, wkick, 

 in tkat fisk, connected tke bottom of tke cavum sinus imparis witk tke vertebra-like depression in 

 tke kind end of tke basioccipital, and I said tkat tkis seemed to indicate tkat tke cavum sinus imparis 

 migkt be tke remnant of tke anterior conical depression of a vertebral body. In Peristedion it is not 

 tke cavum sinus imparis, but tke kind end of tke myodome, tkat kas tke appearance of being suck 

 a depression on tke anterior surface of a vertebral element; and if it be suck a depression, it would 

 offer a rational explanation of tke basioccipital extension of tke myodome. That this extension 

 of the myodome is due simply to the fact that the rectus externus, deriving great advantage from 

 a slight additional posterior shifting of its point of origin, has extensively excavated the basioccipital, 

 has never appealed to me. And if a simple posterior extension of its point of origin is of such con- 

 siderable advantage to the rectus externus, why should it not also be of some advantage to the rectus 

 internus, which muscle, in Scomber and in all of the mail-cheeked fiskes tkat I kave examined, 

 never acquires tkis posterior extension? But, if tkere were a pre-existing depression in tke anterior 

 end of tke basioccipital, its occupation and subsequent enlargement by one only of tke two muscles 

 would seem most natural. Similar reasoning, applied to tke proötic, would account for tke 

 origin of tke proötic part of tke myodome, as will be furtker discussed in tke section devoted to the 

 myodome of fishes. 



