OF THE TELEOSTEAN FISH SEBASTES MARINUS. 509 



septum. This fragmentation of the trabecula? is not uncommon 

 in Teleosts. A somewhat similar process takes place in Gyclo- 

 2)teriis (Uhlmann, 1921). 



Above the nasal septum lies a slightly elongated rostral 

 cartilage. It shows no connection either with the ethmoid or 

 palatine cartilages. 



The parachordals have given rise to a substantial investment 

 of the auditoiy capsules and posterior part of the brain. The 

 notochord has retreated considerably relative to the auditory 

 capsules, and is now bridged over by cartilage. Below and in 

 front of the articulation of the hyomandibular with the auditory 

 capsule a process projects forwards on eitlier side. These probably 

 represent the anterior part of the parachordals. The trigeminal 

 nerve passes out through a notch above them. The auditory 

 capsules are now completely enclosed and the cartilage is continued 

 as a roof over the hinder part of the bi'ain. From the side wall 

 of the brain there runs forward on each side a postorbital process 

 which joins the ends of a transverse bridge, the epiphysial bar. 

 This bar (text-fig. 8) cuts off the posterior dorsal fontanelle, and 

 the fontanelle is divided into lateral halves by a taonia tectum 

 medialis, a thin band of cartilage joining the epiphysial bar with 

 the posterior covering of the cranial cavity. In Salmo, Amia 

 (Pehrson, 1922), and others the epiphysial bar, beiug connected 

 also with the taeniae margiuales, forms in addition an anterior 

 dorsal fontanelle which gradually fills up. This anterior roofing 

 in does not occur in Sehastes. In Amiurus (Kindred, 1919) a 

 permanent anterior fontanelle is formed which, howevei", does not 

 close up, while \\\ Gasterosteus the fontanelle is formed, but the 

 epiphysial cartilage, as is probably the case in Sehastes, becomes 

 disconnected from the ethmoid region. 



It is now necessary to consider how the cranial elements 

 developed from the parachordals have a,risen. It is obvious that 

 there has been a vigorous upwaid growth from the edges of the 

 cartilage which in the 5*5 ram. stage foi'med the base of the audi- 

 tory capsule. This growth has extended so far as to roof over the 

 posterior part oi the cranial cavity. Tlie postorbital processes 

 have evidently grown forwards from the lateral walls of the 

 cranium, but it is difficult to say whether the epiphysial bar has 

 arisen independentlj'^ or whether it has been formed by the joining 

 together of processes growing upward from the ends of the post- 

 orbital processes. Independent formation is the more common 

 condition among Teleosts. The taenia tectum medialis is probably 

 a backward growth from the epiphysial bar. This is how it is 

 formed in Gyclopterus, and in Sehastes the anterior part of the 

 band is broad and strong while the posterior part is very thin and 

 frail. 



The changes undergone by the jaw arches au<l visceral skeleton 

 are almost as great as these displayed by the cranium. The lower 

 jaw is a long, rod-like cartilage bearing coronoid and retro-articnlar 

 processes at the point of its articulation with the quadrate. In 



