42 MB. A. D. IMMS OK THE DENTICLES [Jan. 17, 



gill-raker met with in Polyodon and CetorJdnus might have been 

 derived from them through their spinous portions becoming greatly 

 elongated. As the result of this examination, I has^e found that 

 denticles are of very frequent occurrence both on the branchial 

 arches and on the mucous membrane lining the mouth and 

 pharynx in these Fishes. A brief account of the observations 

 which I have made on these structures is embodied in the present 

 article. I am indebted to Prof. T. W. Bridge, F.R.S., for helpful 

 criticism and for his kindness in placing at my disposal a number 

 of examples of various fishes. To Mr. G. A. Boulenger, F.R.S., I 

 also owe a debt of gratitude for allowing me to examine specimens 

 of several genera of Elasmobranchs in the Collection of the British 

 Museum. My investigation of this subject was commenced about 

 the time when Steinhard's paper was published, and I did not 

 become aware of the latter until my work was neai-ing completion. 

 When I came to read his paper, I found that I had conducted my 

 studies on similar lines and, moreover, that about half the species 

 examined by me had already been investigated by him. As the 

 result of this coincidence, I found it necessary to curtail the 

 present paper considerably below its original dimensions, and, in 

 its emended form, I offer it as a small additional contribvition to 

 a knowledge of these denticles. 



In his paper Steinhard refers to the oral and pharyngeal 

 denticles as " Schleimhautschuppen," or " mucous membi'ane 

 scales," and he describes their form, arrangement, and distribution 

 in a number of cases. The first part of his description treats of 

 their occurrence among the Selachoidei, and the following species 

 of the latter were examined by him : — Heptanchus clnei^eus, 

 Mustelas vulgaris, Carcharias glaucus, Fristvm'its 'tnelanostomus, 

 P. sp., Acanthias vulgaris, Centrophorus sp., ScylUum hurgeri, 

 S. canicula, Galeorhinus japonicus ?, Sphiax niger, and Squatina 

 vulgaris. He points out that from among these twelve species 

 nine out of them possessed " Schleimhautschuppen." In some 

 forms {Heptanchus, Mustelus, Carcharias) the whole of the mucous 

 membrane lining the mouth and pharynx down to the commence- 

 ment of the oesophagus is closely covered with them. In others 

 [Acanthias, Pristiurus sp.) they invest the mucous membrane of 

 the branchial arches, but their distiibution over the rest of the 

 mouth and pharynx is more restricted. In Pi'istiurus melano- 

 stormis, Centrophorus sp., and ScylUum hiLvgeri denticles were 

 only present over the mucous membrane covering the branchial 

 arches, while in Squatina they are confined to the lining of the 

 oral cavity. The second part of the paper deals with Galeorhimis 

 canis and the Batoidei. In the former, he describes the whole of 

 the oral and pharyngeal cavities, together with the branchial 

 arches, as being completely covered with denticles which extend 

 as far back as the commencement of the oesophagus. A similar 

 condition was met with in Pristis perotetti, which he considers in 

 this respect to be the most primitive of the B;vtoidei. The other 

 members of the group which he examined were lihijachohatis 



