OF THE LAEVAL POLYPTEEUS. 321 



On the ventral surface of the auditory region of the cranium, from the point in 

 the middle line where the aorta bifurcates, on either side there extends forwards and 

 outwards a deep groove in which runs the branch of the aorta, being the efferent artery 

 from the hyoid external gill, and receiving a small branch from the first branchial 

 arch. 



Anterior to the bridge of cartilage uniting dorsally the two lateral masses of 

 cartilage of the auditory region, the roof of the cranium is formed entirely of 

 membrane to the front end of the cranial cavity, except for a delicate but continuous 

 bridge of cartilage in the region of the pineal body (PI. XXXIII. figs. 1 & 3, Ej).hr.). 

 This slender bridge expands towards the middle line into a horseshoe-shaped plate 

 (PI. XXXIV. fig. 1, Ep.br.) lying over the vesicle of the third ventricle i. The pineal 

 gland itself lies in the centre of the horseshoe, and is not covered by cartilage but lies 

 in the dermis of the skin (PI. XXXIV. fig. 1, Ep.). 



The lateral walls of the cranium extend forward from the auditory region on either 

 side of the alisphenoid region as continuous vertical plates of cartilage, somewhat 

 dumbbell-shaped in section and perforated by foramina for the iiird, vth, and viith 

 nerves in the thinner middle portion (PI. XXXIII, fig. 1, III., V., VII.). In the 

 sphenethmoid region there is a large lateral fontanelle closed only by membrane 

 through which passes the optic nerve, while above and below this membranous portion 

 there pass the thickened upper and lower cartilaginous borders of the cranial wall, 

 connecting on either side the ethmoidal region with the posterior region of the 

 cranium (PL XXXIII. figs. 1 & 3). A section therefore passing through the middle 

 of the orbital region cuts merely four bars of cartilages — two supraorbital bars separated 

 from one another by a wide superior fontanelle, and two infraorbital bars (trabeculae 

 cranii) separated by a wide inferior fontanelle, the upper and lower of each side 

 being separated by the large lateral fontanelle ; the greater part of the " sphen- 

 ethmoid " bone of Folyptenis is therefore not preformed in cartilage. 



In this region the section of the supraorbital bar of cartilage is V-shaped ; the outer 

 limb of the V is the anterior prolongation of the sphenotic wing, which is here fused 

 with the supraorbital bar (PI. XXXIII. fig. 3, Sph.). In the angle thus formed 

 lies the supraorbital canal of the lateral line system (PL XXXIV. fig. 1, s.o.c). In 

 front of the interorbital region the four bars of cartilage m the side walls of the 

 cranial cavity expand and unite together, forming the anterior wall of the cranial 

 cavity and being fused with the olfactory capsules (PL XXXIII. fig. 1). A canal for 

 the passage of the olfactory branches of the vth and viith cranial nerves penetrates the 



' This vesicle is called by Waldsohmidt the " epiphysis cerebri (pineal gland) " ; in reality, however, it is the 

 homologue of the median vesicle of Lejjidosteus described by Balfour and Parker (Phil. Trans. Roy. Soc. Lond. 

 part ii. 1882). The epiphysis itself lies over and behind this vesicle, and was apparently not found by 

 Waldsohmidt in Pohjpterus (loc. cit.). 



