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Show 1886.] SQUALORAJA POLYSPONDYLA. 533 the two excavations at the base of the rostrum in the fragment shown in Davies's fig. 4 evidently testify to their normal proportions and situation. In a line with the prominence is fixed the base of the rostral spine (r.s), quite at the hinder extremity of the ethmoidal tract; and still more posteriorly, the chondrocranium begins to exhibit considerable lateral compression, though finally widening to a slightly broader occiput. Extending backwards from the antorbital process, the slender postpalatine cartilage (pt.pa) is preserved in most specimens (especially in no. IV. fig. 2 ), but there is some uncertainty as to whether it formed a distinct element. It tapers slightly to its distal end, and the crushing during fossilization has usually imparted to it the deceptive appearance of connection with the hyomandibular. N o postorbital process can be observed, and the circumstances of preservation are probably accountable for the absence of any trace of a fontanelle in the cranial rcof; but there is an interesting V-shaped protuberance (a.v) close to the hinder extremity, evidently representing a fold round the hollow into which opened the aqueductus vestibuli (or ductus endolymphaticus) of each auditory sac. O n the ventral aspect, the parachordal, or " investing mass" (fig. 3, iv.m), is produced posteriorly into a pair of occipital condyles (oc.c), as already noted by Riley; and there is a well-marked median ridge (n), obviously due to the remains of the primitive notochordal sheath. A median foramen (cf) is also somewhat conspicuous, and, if not the result of accident during fossiliza-tion, is evidently the passage for the united internal carotid arteries proceeding to the pituitary body1. Of the mandibular and hyoid arches, the hyomandibular cartilage (figs. 1, 2, hm) is the only portion satisfactorily preserved. In its crushed condition it is seen to extend from each side of the occiput, curving outwards and forwards, and gradually tapering to the distal extremity. In shape it approximates to that of most " Batoidei," being twice as broad proximally as distally, and its apparent continuity with the cranial roof is probably due to the process of fossili-zation. Not a trace of the pterygo-quadrate and mandibular cartilages appears exposed to view ; but the arrangement of the dental plates in the specimen no. III. (fig. 3, t) shows that the two rami of the jaw met at the symphysis in a comparatively acute angle, and were not placed in the same straight line, as is the case in so many living Rays. But the most remarkable feature to be noticed in the skull of Squaloraja is presented in the two pairs of transversely elongated appendages, with reflected ends, arising from beneath the narrow part of the palato-trabecular region. These curious structures are not well shown in our fig. 1 (ci.a, ci.b), but can be studied in their entirety in the large specimens figured by Riley and Davies. The most anterior (ci.a) is the larger, and is completely displayed on both sides of the last-named fossil; its total length is equal to three times the width of the skull at the position where it emerges, 1 See T. J. Parker, ' Zootomy,' 1884, p. 62, fig. 20. |
