1905.] ox THE DEXTICLES OF ELASMOBKANCH FISHES. 41 



one instance 10 mm., of which the proboscis forms only the fifth 

 part, i. e. 2 mm. The ventral hooks lie close up behind the 

 proboscis. The skin is rather thin, and only partially transparent 

 so far as concerns most of the internal organs, but the nerve-cord 

 is clearly visible from the outside. The structure of the papillate 

 bodies is shown in fia:. 5. 



EXPLANATION OF PLATE II. 



Fig. 1. Physcosoma socium (p. 37). a. Dissection showing internal anatonij'. 

 h. Papilla ft-om mid-body. c. Skin of introvert, showing gradual flat- 

 tening of the papilla;. 



Fig. 2. Physcosoma gaudens (p. 38). Papilla from the hind end of the body. 



Fig. 3. P/iascolosoma piriformis (p. 39). a. Dissection showing internal anatomy. 

 b. Papilla from the front end of the body, surface view. c. The same in 

 section. 



Fig. 4. Asjpidosiphon insularis (p. 40). a. Dissection showing internal anatomy. 

 h. Hooks, c. Papilla from the base of the introvert. 



Fig. 5. Thalassema sahinum (p. 40). Skin from the front of the body. 



7. On the Oral and Pharyngeal Denticles of Elasmobraiich 

 Fishes*. By A. D. Imms, B.Sc. (Lond.), Zoological 

 Laboratory, University of Birmingham. 



[Received November 1, 1904.] 



(Plate Ill.t) 



It is well known that in the Elasmobranch Fishes true teeth are 

 carried only in relation with the palato-quadi-ate and mandibular 

 cartilages. Minute denticles, however, may be present in greater 

 or less abundance in many parts of the lining of both the oral 

 and pharyngeal cavities. Very little has been written with regard 

 to these structures, and, although reference is made to them by 

 Hertwig, Popta, and others, the only general description of them 

 is that recently published by Steinhard$. 



I have been led to devote some attention to them as the outcome 

 of an account which I have recently given of the structure of the 

 gill-rakers of the Ganoid Fish Polyodon spathula §. In that 

 paper I suggested that the gill-rakers of Polyodon may perhaps 

 be regarded as scales (or denticles) which have migrated fiom the 

 exterior of the body on to the branchial arches, and have there 

 become greatly modified into long setiform structures. In order 

 further to test the possibility of this suggestion, I have examined 

 examples of species belonging to a considerable number of genera 

 of Elasmobranchs for the purpose of ascertaining whether denticles 

 of any description are present on the branchial arches in those 

 Fishes. Given the presence of denticles on the branchial arches 

 in such forms, it would not be difficult to conceive that the type of 



* Communicated by Prof. T. W. Bridge, F.R.S., F.Z.S. 



t For explanation of the Plate, see p. 49. 



X Archiv fiir Naturgesch. Ixix. Bd. i. 1903, pp. 1-46, Taf. i. & ii. 



§ Proc. Zool. Soc. 1904, vol. ii. pp. 22-35, pi. ii. 



