THE SKULL OF CHIM.KHA. l?,o 



found in my specimens of Chiincera coUiei "ai"ising from the 

 tougli fibrous tissues that cover the ventral surface of the sym- 

 physis of the mandibles, and also from the external surface of the 

 cartilage a. Ptunning latero-posteriorly in a curved line, this 

 ligament is joined first by a ligament coming from the median 

 line posterior to the cartilage a, and then by a ligament coming 

 from the ventral edge of the mandibular labial and to which 

 I'eference has just above been made. The ligament so formed 

 runs posteriorly across the postero-ventral edge of the mandibu]n, 

 in a slight groove in that edge, and then runs upwai^d along the 

 internal surface first of the mandibula and then of the palato- 

 quadrate, and is inserted on a little cartilage which seems to 

 correspond to the spiracular cai'tilage of Hubreclit's descriptions 

 of Chimrera monstrosa, notwithstanding that it lies much farther 

 fi'om the hind edge of the mandibula. This little cartilage is 

 pi-obably a persisting remnant of a mandibular branchial ray 

 such as is frequently found in the Selachii and there currently 

 Ciilled a spiracular cartilage, but it cannot be the homologue of 

 the spiracular cartilage of the Batoidei, that cartilage being the 

 extrabranchial, or siiprapharyngobranchial, of the mandibular 

 arch and being represented, in Chimcera, in the processus oticus 

 quadrati, as already stated. The ligament related to this little 

 cartilage may then represent either certain persisting fibrous 

 tissues of the mandibular arch, or b® a ligament derived fi'om 

 certain fibres of the primitive constrictor of the arch such as ore 

 found in Asti'cqje (Luther, 1909 «, p. 14), and to which I have 

 made reference in a work now in press on the homologies of 

 the muscles related to the visceral arches in the gnathostonie 

 fishes (AUis, 1917 a). If this little cartilage be a persisting 

 remnant of a mandibular branchial i-ay, then the cartilage a 

 would also seem to be such a remnant. Luther (I. c. p. 46) 

 considers the cartilage a to be a chondrification of the membrane 

 in which it lies. 



3. Bostral Processes. 



The three rosti^al processes of the adults of all of the Chima3- 

 roids are said by Garman (1904) to be attached to the chondro- 

 cranium by ligaments "in such a way as to admit of consideraVjle 

 movement of their distal extremities up and down," the evident 

 inference being that Garman did not find, in any of these fishes, 

 the cartilage of these processes continuous with that of the chon- 

 drocranium. Hubrecht, however, shows all three of these 

 processes directly continuous with the cartilage of the chondro- 

 cranium, and he suggests that the median process may be the 

 homologue of the i-ostral process of the tSelachii, and that 

 the lateral processes are probably represented, in the latter 

 fishes, by ligaments. Schauinsland (1903) refers to these pro- 

 cesses, in embryos of Callorhjjnchus, as "mit dem 8chadel fest 

 verbundenen Knorpeln," and in. his figures he shoAvs all three of 

 them as outgrowths of the septal cartilage of his descriptions 



