— 146 — 



profundi and also the encephalic branch 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 Trigia, issuing from the cranial cavitv 

 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 prepituitary portions of the mesial processes of the proötics are replaced by 

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

 no basisphenoid 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 the parasphenoid. 

 The hypophysial fenestra extends nearly the füll length of the myodome, but is much narrower in 

 the basioccipital region than in the proötic. The fenestra is completely closed by the underlying 

 parasphenoid, the myodome not opening, posteriorly, on the ventral surface of the skull. The roof 

 of the basioccipital portion of the canal is formed by a thin plate of bone, which separates this part 

 of the myodome from the overlying cavum sinus imparis. The cavuni sinus imparis extends poste- 

 riorly slightly further 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 the hind end of the bone, in such sections, is lined 

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

 the bone. A thinner layer of similar bone lines the deeper conical depression, in the anterior end 

 of the bone, that forms the hind end of the myodome. The ends of these two cones approach each 

 other, and the dense bone lining them is continued, in the middle line of the bone, from one cone 

 to the other. In Scomber I described ('03, p. 102) a similar but much less pronounced line, which, 

 in that fish, connected the bottom of the cavum sinus imparis with the vertebra-like depression in 

 the hind end of the ba.sioccipital, and I said that this seemed to indicate that the cavum sinus imparis 

 might be the remnant of the anterior conical depression of a vertebral body. In Peristedion it is not 

 the cavum sinus imparis, but the hind end of the myodome, that has the appearance of being such 

 a depression on the anterior surface of a vertebral dement; and if it be such a depression, it would 

 offer a rational explanation of the basioccipital extension of the 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 fishes that I have examined, 

 never acquires this posterior extension? But, if there were a pre-existing depression in the anterior 

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

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

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

 myodome of fishes. 



