128 MR \y. K. PARKER ON THE STRUCTURK AND 



Plate (22) with the others. Cijclorhamphus is its nearest ally, by far, and in this 

 comparison we liave dug deeper for evidence of kinship than in any comparison that 

 could be made of the skull in the adult. 



Small hind legs have budded out in this larva, and as a correlate of these growths 

 three investing and two intrinsic bones have appeared in the skull ; these lift it out 

 of the sphere of simple "chondrocrania." 



The notochord («c.) is still of considerable size, but the copious development of 

 hyaline cartilage has already obliterated several morphological landmarks. 



The occipital condyles (occ.) are now well formed, and the arch itself is very wide, 

 rather low, and has a huge doorway {f.ui.). The large oval ear-sacs are completely 

 chondrified, and are confluent with the basis and tegmen cranii ; they abort its walls 

 largely, growing into the sides of the skull ; the old lines of junction can, however, be 

 traced. 



The cranium proper is very wide, es])ecially behind, and the tegmen is developed, 

 already, nearly as much as in Sharks — more than in most Skates ; it has two lesser 

 fontanelles in it {fo'.). The interorbital part is wide, gently bulging, and gradually 

 narrowing to the ethmoidal region, which is now in the act of closing in ujjou the fore 

 part of the brain capsule. Above (fig. 2) the prmcipal fontanelle {fo.) is only a 

 quarter the size of the general tegmen of cartilage, and in front it is unenclosed — the 

 lateral halves of the primaru ethmoidal region of the skull-wall have not met over the 

 brain cavity. Also the unfinished state of the skull is seen still further m this, 

 namely, that the "perpendicular ethmoid" {p.c) is still a mere oval upgrowth of 

 cartilage — a tuberous ascending development of the intertrabecular tract which is still 

 visibly distinct from the ti'abeculaj, all the way from the pituitaiy floor to where the 

 trabeculse become again free in front as the " cornua" [c.tr.). 



In front of that little swelling of cartilage (fig. 2, p.e.) the foundation of the septum 

 nasi is seen in the narrow foremost part of the cartilage which conjugates the two 

 trabeculse. 



We get here additional light upon the vegetative growths of cartilage that finish, 

 by closing in, the cranial capsule, in front. Where the walls have most converged in 

 front they there suddenly grow out as the "ethiiioidal wings'' {ale.), running by con- 

 tinuous growth of cartilage into the pterygo-palatine bands {p.p;/.). 



The olfactory nerves (1) escape here from inside the thickening wall which gives ofl" 

 the wings ; in the middle the mesethmoidal " tuber" {p.e.) becomes a flatter structure, 

 and grows into a vertical partition wall between the nerve-outlets, the foremost of 

 the three sense-capsule fenestrcB. The right and left roofs — growths of the ethmoidal 

 wall — unite with each other and with the middle wall ; and besides this, in front of 

 the cranial cavity, each trabecula at its outside develoi)S a process of cartilage, similar 

 to, but less than, the median intertrabecular tuber. These three, growing round the 

 olfactory nerves, finish the round passages (fenestra)), and, with the lateral ethmoidal 

 walls (growing into the roof), close in the skull-building. 



