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Show 32 PROF. T. J. PARKER ON CARCHARODON RONDELETII. [Jan. 18, of comparing the skull with that of the closely allied Porbeagle (Lamna cornubica), I give upper, under, and side views of the cranium of both Sharks, drawn to the same absolute length ; in the side views the jaws also are shown (Plates IV. and V.). Haswell says, " On comparing the skull of Oarcharodon with a dried skull of Lamna cornubica, I can find little difference between the two." As a matter of fact the differences between the two crania are by no means inconsiderable, as Haswell would no doubt have found if his skull of Lamna had not been distorted by drying. The main differences are dependent upon the much greater proportional size of the rostrum and of the orbits in Lamna. In Carcharodon the dorso-lateral arms of the rostrum (first labial cartilages, W . K. Parker1) are slightly curved, with a downward concavity, while the median ventral arm (prenasal cartilage) has a strong downward convexity, so that the three bars meet at a wide angle. All three bars are broad at tbe base, but narrow considerably in front, and are but slightly calcified, the distal portion of the entire rostrum being composed wholly of cartilage. In Lamna, on the contrary, all three rostral bars have a marked sigmoid curve, and meet with one another at very acute angles. They are also much longer proportionally than in Carcharodon, much thicker, and are covered externally with a close mosaic of bony matter. As already remarked, the eyes, and consequently the orbits, are proportionally much larger in Lamna than in Carcharodon; as a result of this, the orbital roofs (sup.orb.pl) in Lamna are strongly arched both antero-posteriorly and laterally, and the infraorbital plates (inf.orb.pl) inclined downwards at their outer ends. The whole cranium also, and especially the basal plate (i. e. the basis cranii proper plus the infraorbital plates), is much narrower than in Carcharodon (cf. figs. 3 and 4), and the parotic (p.ot.pr) and postorbital (p.orb.pr) processes are less prominent. In the foetal specimen (Plate VIII. figs. 24 and 25) the rostrum is very slender, and its ventral or prenasal bar is perforated distally by a foramen. The anterior fontanelle (font.) is very large, and allows the cerebrum to be partly seen in a view from above. The auditory capsules are very prominent, and show clearly the elevations for the semicircular canals. The supraorbital plate is hardly developed, and the infraorbital plate is quite narrow. To the outer surface of the auditory capsule of the foetus, dorsad of the hyomandibular facet, a small rod of cartilage (Pl. VIII. fig. 26, spir.cart.) is attached by fibrous tissue. This appears to be the dorsal segment of the mandibular arch or spiracular cartilage. Unfortunately the specimen had been dissected by one of my assistants, as a help to the articulation of the adult skeleton, before I observed this cartilage, so that I was unable to make out its relations to the spiracle. No corresponding structure was found in the adult, but 1 " O n the Structure and Development of the Skull in Sharks and Rays," Trans. Zool. Soc. vol. x. (1877) p. 189. Parker and Bettany, ' Morphology of the Skull,' p. 35. |
