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Show 532 MR. A S. WOODWARD ON [Dec. 7, the fact of its occurrence in more than a single type, leads to an interesting speculation, suggested to m e by Professor Parker. Compared with the distinct anterior intertrabecular cartilage, which forms the axis of the rostrum in such primitive fishes as the glutinous Hags (Myxine) \ scarcely the slightest difference in form can be noted; and it seems not unlikely that we are here concerned with an admirable illustration of tbe principle, that the contours of superficial structures appended to the cranium are frequently determined, in the main, by the shape of the fundamental cartilages to which they are attached. The fossils, of course, do not permit a determination of the complete form of the intertrabecular cartilage in any of these types, or of its primitive distinctness ; but the slight glimpses that can be obtained are rather favourable than otherwise to such a conclusion. Of the skin itself in Squaloraja, only indefinite patches remain, and no small shagreen granules appear to have been developed in it; but the sharp lateral edge, both of the rostral and caudal regions, is strengthened by a series of minute calcified rings (fig. 1, d), evidently quite similar to those stiffening the boundaries of the snout in the living Pristiophorida?. Cranium and Mandibular and Hyoid Arches.-Nearly all the large fossils under consideration reveal facts of more or less interest in regard to the structure of the skull; nos. I., IL, and IV., with Riley and Davies's specimens, present the dorsal aspect, while no. III. and the fragment shown in Davies's plate, fig. 4, afford some particulars as to the conformation of the ventral surface. The palato-trabecular region (figs. 1, 2, pa.lr) extends far forwards in its present crushed condition, and from the centre is produced the long narrow intertrabecular cartilage (i.tr) forming the axis of the snout. From each anterior outer angle of this region there also arises a more slender forwardly directed cartilage (pr.pa), which gradually tapers to an incurved point, as admirably shown in the left side of no. IV. (fig. 2). This prolongation evidently served to stiffen the edge of the base of the snout, exactly as its well-developed homologue in the living Pristiophorus; and there can be little doubt that it represents a definite prepalatine element, such as has not hitherto been recognized in the skulls of the Selachian order. Its form is almost identical with that of the corresponding cartilage in the Myxinoids, as will be at once seen on referring to Prof. Parker's beautiful figures of Myxine and Bdellostoma2; in these fishes, indeed, the process serves a similar purpose, being likewise placed to strengthen the sides of the rostrum. Immediately behind the origin of the prepalatine " horns," the lateral boundary of the palato-trabecular region gradually curves inwards for some distance, and then as slowly outwards again until it forms a well-marked antorbital prominence ; but the olfactory capsules, in their fossilized state, are totally unrecognizable, though 1 W. X. Parker, "On the Skeleton of the Marsipobranch Fishes-Part I. The Myxinoids," Phil. Trans. 1883, pi. x. fig. 2. 2 \V. K. Parker, loc. cit. pi. x. figs. 1-3 and pi. xvii. figs. 1-3. |