1905.] OF ELASMOIJRANTJn FISHES. 49 



ancestral Yertebiutes oi^ for what pai-ticulav mode of feeding tlie 

 structure of their mouth was adapted. It is highly probable 

 that some light might be thrown on the primitive use of the oral 

 denticles, if one knew the precise nature and disposition of the 

 skeletal structures which bounded the oral cavity in the primitive 

 Vertebrates before the anterior branchial arches ceased to be 

 purely gill- bearing and had acquired the special characters of jaws, 

 as seen in the most primitive of existing Gnathostomata. In the 

 primitive Vertebrata it is possible that the seizure, holding, or 

 perhaps even the crushing of the food mf\y have been effected 

 by the movements of the ventral portions of the arches towards 

 the roof of the oral cavity, after the fashion of the hypopharyn- 

 geal teeth in connection with the hinder branchial arches in many 

 Teleosts. If there be any truth in this suggestion, it will not be 

 difficult to appreciate the physiological value of an extensive 

 distribution of denticles over the gi'eater part of the oral and 

 pharyngeal mucous membrane in the primitive Vertebiates. 

 Y/ith the evolution of special jaws at a latei' period, the functional 

 denticles would naturally tend to become restricted to them and 

 constitute ordinary teeth, leaving, howevei-, the residue of the 

 stomodeal invasion of dermal denticles to become pharyngeal teeth, 

 or gill-rakers, or to remain as vestigial structures, or to vanish 

 altogether. 



EXI'LANATIO>f OF PLATK III. 



The figures are all ninp;iiifie(l about 80 tiuies nnd are from preparations exainineil 

 in g-lycerine, which, in the case of tio-s. 1— i, rendered them siitiiciently transparent 

 J'or the internal structure of the denticles to be seen. 



The spii.ou-i portions of the denticles are directed towards the caudal extremity (f 

 the fish. 



Beferenco Letters. 



^/.j.». = basal plate ; rf.^ = dentine tubuli ; !«.»?. = niucous membrane ; 

 j;.c. = pulp-cavity. 



Fi.^'. 1. A piece of the lining from the Hoor of the oval cavity of Ilustelus Icevix 

 (p. 44), showins- the very closely arranged denticles. 



Fig. 1 a. A single denticle from the same, viewed laterally. 



Fig. 2. A strip of the mucous membrane from the floor of the pharynx of Galevji 

 canis (p. 44). The denticles are seen to have a regular and orderly arrange- 

 ment, but are not so closely disposed together as in Mii$teli(s. 



Fig. 2 a. A single denticle of Galens, viewed laterally. 



Fig. 3. A portion of the mucous membrane lining the floor of the pharynx of 

 Notiilanus cinereus (p. 41). 



Fig. 4. A portion of the mucous membrane from the pharynx of AcaiUhias vulgaris 

 (p. 45), taken from where the last two branchial arches join the iloor of tlu 

 same. 



Fig. 5. A piece of the lining of the roof of the oral cavity of lihina sqiiatina (p. 45). 



Fig. 5«. A single denticle from the same, viewed laterally. 



Fig. 6. A strip of the mucous membrane lining the roof of the pharynx of Eain 

 clavata (p. 46), taken from near the last two branchial arches. In thi< 

 species and in lihina the denticles have lost their regular arrangen^ent and 

 are scattered over the mucous membrane as if at random. 



Proc. Zcol. Soc— 1905. A^ol. I. No. IV, 



