OCR Text |
Show 42 MR. A. D. IMMS ON THE DENTICLES [Jan. 17, gill-raker met with in Polyodon and Cetorhinus might have been derived from them through their spinous portions becoming greatly elongated. As the result of this examination, I have 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 011 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. (i. 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 nearing 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 contribution to a knowledge of these denticles. In his paper Steinhard refers to the oral and pharyngeal denticles as " Schleimhautschuppen," or " mucous membrane 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 cinereus, Mustelus vulgaris, Carcharias glaucus, Pristiurus melanostomus, P. sp., Acanthias vulgaris, Centrophorus sp., Scyllium burgeri, S. canicula, Galeorliinus japonicus ?, Spinax 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 commencement of the oesophagus is closely covered with them. In others (.Acanthias, Pristiurus sp.) they invest the mucous membrane of the branchial arches, but their distribution over the rest of the mouth and pharynx is more restricted. In Pristiurus melanostomus, Centrophorus sp., and Scyllium burgeri denticles were only present over the mucous membrane covering the branchial arches, while in Squatina they are confined to the lining of the oi*al cavity. The second part of the paper deals with Galeorhinus 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 Batoidei. The other members of the group which he examined were lihynchobatis |