1879.] 



Skull and its Nerves in the Green Turtle. 



341 



sules, and then spread into dilated out-turned "cornua." By that 

 time another pair of para-chordal cartilages has appeared behind the 

 first, and in several species of TJrodeles a much smaller pair behind 

 them. I do not find more than a very delicate layer of mesoblast 

 sheathing, the cephalic part of the notochord ; and the inter-tra- 

 becular cartilage in front of it merely fills the interspace of the tra- 

 becule in the uasal region ; only in Siren and Salamandra do I see a 

 short "pre-nasal" rod. 



In most of these types the two last-mentioned being exceptional, the 

 para-chordal part of the trabecule is absorbed, and only a selvege of 

 the para-chordal, proper, remains inside the ear-capsules, and at the 

 occipital condyle. 



In these types the basal cartilage does not grow up into the cavity 

 of the mid- brain as a " post-clinoid " wall, but this wall is very well 

 seen in the dog-fish (Scijlliuni). 



In the Batrachia ("Anura") the trabecule embrace the apex of 

 the notochord less, and develop their cornua sooner ; the rest of the 

 basal plate, behind, is somewhat later in its appearance. They have 

 a continuous growth of the inter -trabecular band, which is a flat floor 

 finishing the skull below ; between the eyes, and between the nasal 

 sacs it grows into a wall ; in front, this tract fills in the angle between 

 the trabecular cornua, and often (as in the tree-frogs) sends forth a 

 distinct pre-nasal rod, like that of the sharks. 



The Anura have a slight post-clinoid wall ; their skull on the whole, 

 when finished, is intermediate between that of the Selachians and the 

 Urodeles. Goette mentions the cartilaginous sheath of the cranial 

 notochord in Bombinator ; I find it very massive in the larve of 

 Pseudis (when the limbs are just appearing) ; but, as a rule, this 

 sheath is very thin in tadpoles. 



In Teleostei (" Salmon's Skull," Plates 1 — 8) the investing mass 

 chondrifies first, and is relatively much larger than the trabecule : 

 they are only sub -distinct, and soon coalesce ; the trabecule grow very 

 rapidly, so that at the time of hatching there is a very massive 

 j?ro-chordal tract. 



By the middle of the second week after hatching the pro-chordal 

 tracts are quite distinct from the jpctra-chordal, and lie on them 

 obliquely. 



By the middle of the first summer the tracts show, as far as I can 

 see, no sign of their separation ; but in the adult salmon the trabecule 

 only reach to the front of the pituitary space, whilst the anterior 

 half of the para-chordal cartilage receives its bony matter from their 

 " prootics." 



In the salmon the middle ethnoid is at first feebly developed, growing 

 down, as a keel, from the " tegmen cranii ;" and in the adult all we 

 see of the inter -trabecular point is a short wall between the lateral 



