512 MR. N. A. MAOKINTOSll ON THE CIIONDROCRANIUM 



become more rod-like. It is possible that the expansive form of 

 the ceratohyal is correlated with the fact that the gill-bars reach 

 far forward, in that its position suggests that it serves as a 

 protection to the anterior parts of the bars, the posterior parts 

 being of coui'se protected later by the development of the 

 opercular bones. The anterior junction of the ceratohyals is 

 surmounted by a short tongue of cartilage, and there passes back 

 a median bar, the basi-branchial, corresponding to the copula 

 communis (text-fig. Si). To this bar ai-e attached the fiist four 

 branchials. The fifth pair are free but articulate with one 

 another at their bases. The third pair is peculiar in that the 

 base of each is produced forwards in two downwardly directed 

 processes. The ceratobranchials are very long and fairly 

 stout near their articulation with the basibranchial ; further 

 back they become very slender, but before meeting the epil>ran- 

 chials they become thicker again. Perhaps the most important 

 development of the branchial elements lies in the appear- 

 ance of the epibranchials. These aie possessed by the first 

 four bars but not by the fifth. They are short in comparison 

 with the ceratobranchials and of a slightly irregvdar, twisted 

 shape. 



Here again it is difficult to fill in any details during the 

 preceding period of development. This, however, is not of great 

 importance as the branchial bars have not undergone any striking 

 changes beyond the addition of the fifth bar, the epibranchials, 

 and a few subsidiary elements. 



4. Summary. 



1. There is a pi-ecocious development of the visceral skeleton. 



2. In at least the earlier stages there is no sign of a fenestra 



hypophyseos. 



3. The cranial flexure is retained for a considerable period after 



hatching. 



4. In the 5*5 mm. stage the hyomandibular, symplectic, and 



quadrate are represented by a simple temporal cartilage. 

 This appears also to be a single piece of cartilage in the 

 earlier stages. 



5. The upper jaw does not appear until a compai^atively late stage, 



and there is evidence that it aiises independently and grows 

 back to meet the quadrate. 



6. The anterior part of the trabeculae gives rise to an interorbital 



septum of rather limited area. 



7. There is a free rostral cartilage situated above the nasal 



septum. 



8. The lower jaw is Jong and exhibits prominent coronoid and 



retro-articular processes. 



9. The hyomandibular is club-shaped and very long. Its length 



is correlated with the special depth of the; head. 



