r()2 The Holocephali, or Chimaeras 



gium and without recognizable analogue of the three large 

 cartilages seen in the sharks, the propterygium, mesopterygium, 

 and metapterygium. In the mouth, instead of teeth, are de- 

 veloped flat, bony plates called tritors or grinders, set endwise 

 in the front of the jaws. The giUs are fringe-like, free at the 

 tips as in ordinary fishes, and there is a single external opening 

 for them all as in true fishes, and they are covered with a flap 

 of skin. These structures are, however, quite different from 

 those of the true fishes and are doubtless independently de- 

 veloped. There is no spiracle. The skin is smooth or rough. 

 In the living forms and most of the extinct species there is a 

 strong spine in the dorsal fin. The ventral fin in the male has 

 complex, usually trifid, claspers, and an analogous organ, the 

 cephaHc holder, is developed on the front of the head, in the 

 adult male. This is a bony hook with a brush of glistening 

 enameled teeth at the end. The eggs are large, and laid in 

 oblong or elliptical egg-cases, provided with silky filaments. 

 The eggs are fertihzed 5&tegr they are extruded. Mucous chan- 

 nels and lateral line are highly developed, being most complex 

 about the head. The brain is essentially shark-like, the optic 

 nerves form a chiasma, and the central hemispheres are large. 



The teeth of the Chimasras are thus described by Woodward, 

 vol. 2, pp. 36, 37: 



" In all the known families of Chimasroids, the dentition 

 consists of a few large plates of vascular dentine, of which 

 certain areas ('tritors') are specially hardened by the depo- 

 sition of calcareous salts within and around groups of medullary 

 canals, which rise at right angles to the functional surface. In 

 most cases there is a single pair of such plates in the lower jaw, 

 meeting at the symphysis, while two pairs are arranged to 

 oppose these above. As a whole, the dentition thus closely 

 resembles that of the typical Dipnoi (as has often been pointed 

 out) ; and the upper teeth may be provisionally named pala- 

 tine and vomerine until further discoveries shall have revealed 

 their precise homologies. The structures are sometimes de- 

 scribed as 'jaws,' and regarded as dentaries, maxillae, and 

 premaxilliE, but the presence of a permanent pulp under each 

 tooth is conclusive proof of their bearing no relation to the 

 familiar membrane-bones thus named in higher fishes." 



