32 PROF. T. H. HUXLEY ON CERATODUS PORSTERl. [Jan. 4, 



This would seem to imply that the chondrocranium of vertebrated 

 animals is formed by the coalescence of parts analogous to those which 

 compose the bony skull of osseous fishes. As a matter of fact, how- 

 ever, the chondrocranium is never formed of such elements, but is 

 built up, at a very early stage of embryonic development, by the 

 union of primitively distinct parachordal, otic, and pleural elements*, 

 which in no way correspond with the bones of the teleostean skull. 

 And when Dr. Giinther subsequently (l. c. p. 522) speaks of " three 

 groups of superficial labial cartilages," " the upper labial," the " su- 

 praorbital," and the "lower labial," the discrimination of such car- 

 tilages must, I think, be regarded merely as regional anatomy ; and 

 it must not be supposed that they have any thing to do with the 

 cartilages to which the same names are applied in other fishes, several 

 of which exist in Ceratodus, and will presently be described. 



When the osseous and merely fibrous structures are carefully re- 

 moved, I find that the chondrocranium (figs. 4, 2, and 7) consists of a 

 continuous cartilaginous mass, the interorbital region of which is much 

 narrower than any other part, produced inferiorly and laterally into 

 two stout suspensorial or palato-quadrate processes, with the pulley- 

 shaped ventral ends of which the strong Meckelian cartilages are articu- 

 lated. Anteriorly the orbits are bounded by the antorbital processes 

 (A.O), which curve downwards in front of the eye. From these ant- 

 orbital processes the cartilage is continued forwards to form the evenly 

 curved roof of the ethmoidal region and its contained nasal chambers, 

 and, bending down on all sides, ends in a free edge, which is slightly 

 concave opposite each anterior nasal opening (a«, fig. 4), and much 

 more deeply excavated opposite the posterior nares (pn, fig. 4). The 

 small process (a) which lies between the two excavations in question 

 is connected by a strong fibrous band with the antorbital process 

 (A. O), and thii?, by the ossified bar described by Dr. Giinther, with 

 the postorbital process (P-O) ; but these structures have nothing to 

 do with the chondrocranium. Behind the orbits, the skull suddenly 

 widens out into two broad periotic masses, which lodge the auditory 

 labyrinth. Anteriorly and ventrally these processes are continued 

 into the suspensorial pillars (Qu) ; while behind they pass into thin 

 but wide cartilaginous plates (O, C, fig. 4), which roof over the 

 chambers in whi^^h the branchiae are lodged. 



I thought at first that Ceratodus had no labial cartilages ; but 

 at length I discovered two small upper labial cartilages in their right 

 places, namely in the region of the nostrils. 



One of them (1 , figs. 4 and 5) lies in the roof of the mouth, just 

 in front, and on the inner side of, the posterior nasal aperture. It is 

 fixed to the mesethmoidal cartilage (M.E) by fibrous bands, and is 

 broader behind than in front. The inner edge of this cartilage is 

 concave, the outer convex, and it has a nearly horizontal direction. 



The second cartilage (2, figs. 4, 5, 7) is stouter, and lies behind, 

 and on the outer hide of, the posterior nasal aperture. Its dorsal 

 end is attached to the base of the skull and anterior part of the 



* "On the Structuie of tlie Skull &c. of Menohranchts lateralis" P. Z. S. 

 1874. 



