XX] HOLOCEPHALI 47 1 



the gill-slits. The gills are, however, still borne on the walls of 

 sacs, but these are much shallower than the gill-sacs of Elasmo- 

 branchii. The snout or prae-oral part of the body is much reduced 

 in size and supported only by a single rod of cartilage. 



The scales have almost entirely disappeared and are represented 

 only by the great spine, the so-called icthyodorulite, which 

 stiffens the front edge of the dorsal fin, by the teeth and by the 

 prickles on a peculiar tentacle situated on the snout of the male. 

 The teeth are confluent, forming ridges of dentine covered with 



X 



FIG. 230. Skull of a male Ghimaera monstrosa. After Hubrecht. 



1. Nasal capsule. 2. Cartilaginous appendage to the ethmoid region, 



representing the rostrum of Elasmobranchii. 3. Erectile appendage 



beset with placoid scales. 4. Foramen by which the ophthalmic nerves 

 leave the orbit. 5. Foramen by which the ophthalmic branch of the 



Vth nerve enters the orbit. 6. Auditory capsule. 7. Interorbital 



septum. 8. Meckel's cartilage articulating with an outgrowth from 

 the posterior part of the palato-pterygo-quadrate cartilage. 9. Teeth. 

 10. Labial cartilage. n, in, v, vii, ix, x. Foramina for the passage 

 of cranial nerves. 



enamel. Of these there are a pair in the lower jaw, called dentary 

 plates, and two pairs in the upper, termed vomerine and palatine 

 plates respectively, placed one behind the other. Each plate has 

 certain areas, where the dentine is especially thickened, called 

 tritors. The arrangement of these tritors is used in classifying 

 the fossil species. The peculiar tentacle on the head of the male 

 (3, Fig. 230) arises from a pit situated in the middle line of the 

 snout, and bears sharp tooth -like scales at its tip, and is used 

 to grip the dorsal fin of the female during copulation. 



