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Show 578 DR. W. G. RIDEWOOD ON M E HYOBRANCHIAL [May 18 enclosed. The lateral foramen of the adult skeleton is thus not due to a secondary fenestration in a continuous plate of cartilage as might be supposed, but owes its origin to the closure of a deep notch or sinus. By a similar process of overgrowth of marginal cartilage the hyoglossal sinus (hgs., fig. 9), constant in all tongued Anura, is, in Pelodytes, very nearly converted into a foramen such as occurs in the Aglossa. The enclosing cartilages (pa.) are secondary additions to the most anterior parts of the hyoidean cornua (see figs. 8, 7, and 6), and in some specimens actually overlap one another. The posterior and anterior thirds of the hyoglossal space are closed by membrane, while the middle third transmits the large hyoglossal muscles. If we disregard for the moment the detached parts of the hyoidean cornua, the outline of the hyobranchial skeleton is definitely elliptical in shape, the continuity of the ellipse bein^ broken in six places. There is nothing remarkable about the tbyrohyals (t, fig. 9 ); they are broadest behind and are narrowest at two thirds of their length from the posterior end. The cartilaginous processes (jrpl., fig. 9) running parallel to the thyro-hyals on their external side are more strongly developed than is usual in Phaneroglossa, and their swollen extremities touch the circumscribing ellipse. The deep notches in front of and behind this process are closed by imperforate membrane. On the ventral surface of the basal plate or body of the hyoid is a curious splint-bone (v, fig. 9), consisting of a short central transverse bar, from the extremities of which project a pair of long antero-lateral processes and a pair of short tapering postero-lateral horns. The extremities of the latter extend along the venlral surface of the anterior ends of the thyrohyals. The ossification is partly buried in the hyoglossus muscle, some of the fibres of which pass between the bone and the basal plate. The bone is attached to the rest of the hyobranchial skeleton only by its extremities and is readily dissected off. It is not an ossification of the cartilage of the basal plate like the paired and frequently unsymmetrical ossification of Bombinator, but rather corresponds with the V-shaped ventral bone of Alytes and the paired ossification of Discoglossus l. There are numerous muscles in relation with the hyobranchial skeleton of Anura, and, in making a comparative study of this portion of the skeleton in different genera, the evidence which the muscles afford towards the recognition of homologous skeletal parts is not infrequently of the greatest value. It is only in the Frog (Rana) that the muscles of this region of the body have been studied with any degree of precision, and, since the hyobranchial skeleton of this genus is most familiar to anatomists, I have instituted a comparison between the areas of muscle insertion in the hyobranchial skeleton of Pelodytes and Rana. The muscles of 1 Parker (12. pt. 20. fig. 10) does not show these splint-bones; but, as lie himself has since admitted (Phil. Trans. Boy. Soc. vol. 173. 1882 (1883), p. 139), the specimen there figured is one of liana esculenta, and not of Discoglossus. |