1905.] or ELASMOBRAXCH FISHES. 43 



djeddensis, Haja clavata, Torpedo marmoraia, and Trygon sejjhen, 

 together with Chimccra monstrosa. The paper is accompanied by- 

 numerous figures which show the varied forms assumed by these 

 denticles. Those of HeptancJms in nowise differ in their form 

 from the placoid scales of the skin in that genus. Those of 

 Mustelus and Pristis are regarded as being intermediate in form 

 between the teeth of the respective species on the one hand and 

 the placoid scales on the other. In other genera their resemblance 

 to the placoid scales becomes more remote. The author briefly 

 refers to the function of these structures, and suggests that they 

 may serve in grinding up the food. For that purpose a slight 

 side rubbing-movement of the jaws and gill-apparatus might 

 suffice. By means of such a motion the scaly mucous membrane 

 W'ould act like two rough surfaces. Mention is also made of the 

 difficulty to account for the presence of such structures over an 

 area like the pharynx, which is of hypoblastic origin. He shares 

 the view of Hertwig that their presence in that i-egion is more 

 likely due to a migration of the ectoderm rather than to the 

 possibility of the hypoblast having acquired a scale-forming 

 capacity. 



My observations on the presence and distribution of oral and 

 pharyngeal denticles have been made on specimens belonging to 

 eighteen genera of E]asmobranchs. 



A. SeL ACHOIDEI*. 



Fam. Oarchartid^. 



1 . Carcharias glaucus Rond. — In an individual which measured 

 39 cm. in length, denticles were present over the floor of the oral 

 cavity and along the pharyngeal margins of the branchial arches. 

 Steinhard remarks that in an example of this species which he 

 studied, measuring 46 cm. long, the whole of the cavity of the 

 mouth and pharynx, together with the branchial arches, were 

 covered with denticles which extended as far back as the com- 

 mencement of the oesophagus. 



2. 0. LATICAUDUS MuU. & Henle. — An examination of four 

 specimens of this species, varying in length from 18-42 cm., 

 showed that the oial cavity was closely covered with minute 

 denticles, which extended backwards to about the level of the first 

 gill-clift. In the region of the pharynx they w^ere found only 

 along the inner or concave margins of the branchial arches. In 

 a fifth example of this fish, 26 cm. long, no denticles w^ere to be 

 detected except a few along the bi-anchial arches. 



3. Sphyrxa (Zyg.ena) malleus Risso. — In this species the whole 

 of the lining of the oral and pharyngeal cavities, as far back as 

 the entrance into the oesophagus, was covered with a complete 



* In this classifiL-atioii I liave followed Guiilhcr, Brit. JMiis. CHit. Fislics, vol. viii. 

 1870. 



