OCR Text |
Show 30 PROF. T. J. PARKER ON CARCHARODON RONDELETII. [Jan. 18, Hasse l and a briefer account of the entire skeleton is given by Haswell2. I shall therefore confine myself to a few points which do not appear to have been insisted on, mentioning especially such as seem to be important for comparison with Lamna. a. The Vertebral Column.-In specimen A there are about 182 vertebral centra; at the end of the tail it becomes difficult to count them accurately. The centra agree with Hasse's description, except that I do not find the difference in the disposition of the radiating lamella, of bone which that author gives as distinguishing the trunk- from the tail-vertebrae. Hasse describes and figures only two very thick dorsal lamellae in the caudal vertebrae, between the origin of the neurapophyses: in m y specimens there are three or four comparatively thin lamellae as in the trunk-vertebrae (cf. figs. 13 and 14, Plate VI.). One point, not very clearly brought out by Hasse is the extreme irregularity in the segmentation of the neural tube and of the haemal tube or ridges. These are, in the embryo, continuous cartilages 3, which undergo segmentation at a later stage than the centra, becoming divided into vertebral portions, the neur- and hsemapophyses, and intervertebral portions, the interneural and interhaemal pieces, or intercalaria. The irregular way in which this segmentation takes place in Carcharodon is very striking, and is well shown in figs. 7 and 8 (Plate V.), the former representing a portion of the neural tube seen from above, the latter a portion of one of the haemal ridges seen from below, Occasionally the distal portion of a haemapophysis becomes segmented off, forming a rib (fig- 8, r). Another matter not touched upon by Hasse is the modification undergone by the vertebral column at its anterior and posterior extremities. Anteriorly there is no clear line of demarcation between skull and vertebral column. At the level of the third vertebral centrum (fig. 6, vert.cent. 3) the neural tube meets on each side with the corresponding haemal ridge, forming a continuous lateral investment of cartilage over the first two centra, which are thus only visible from beneath. The continuous lateral cartilage thus formed passes insensibly into the exoccipital region of the skull, while the first and second centra pass into the basioccipital region, and the neural tube into the supraoccipital region. Thus, when the skull is separated artificially from the vertebral column in such a way as to leave intact the great parotic processes (fig. 1, p.ot.pr), the plane of section passes naturally between the second and third vertebral bodies, aud the first two centra appear to be imbedded in the basis cranii (fig. 3). It is also worthy of notice that in the first few vertebral segments the intercalary pieces (fig. 6, i) are small triangular processes 1 Das natiirliche System der Elasmobranchier. Jena, 1879. 2 " Studies on the Elasmobranch Skeleton," Proc. Linn. Soc. N. S. W . vol. ix. (1884) p. 3. 3 Balfour, ' Comparative Embryology,' vol. ii. p. 454. (Memorial edition, vol. iii. p. 550. |