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Show 1887.] PROF. T. J. PARKER ON CARCHARODON RONDELETII. 31 inserted between the bases of adjacent neural arches, the latter (n.a) forming the whole dorsal region of the neural tube; whereas in the remaining greater part of the column the intercalaria form actual interneural arches. The tail-fin contains nearly three-fourths as many vertebrae as all the rest of the column, the 107th centrum being the first of the tail-fin (Plate VI. fig. 12, vert.cent. 107), recognizable by being the first to develop a haemal spine. In this and the five following vertebrae the haemal spine (hcv.sp) is a separate cartilage, quite distinct from the haemapophyses ; in the remaining tail-vertebrae the two are continuous. The haemal spines gradually increase in length to the 120th vertebra, and then undergo progressive diminution : the longest, in specimen A, is 10 cm. in length. The tail-fin is thus supported ventrally by haemal spines : the small portion lying dorsad of the vertebral column, on the other hand, has its framework constituted by a series of cartilages (ptg) which are evidently not neural spines but pterygiophores * or cartilaginous fin-rays. These have no definite relation to the vertebral segments, one of them sometimes corresponding to a single vertebra, sometimes to two, and sometimes to three. Hasse remarks that while intercalaria are absent in the haemal tube in the caudal region, they are present in the neural tube, which has therefore double neural arches as in the trunk region. This is true only as far back as the 130th vertebra (vert.cent. 130), caudad of which intercalary pieces are absent and the neural arches consequently single. In the 167th (vert.cent. 167) and following vertebrae, the neural and haemal arches are united with one another on each side by a vertical bridge of cartilage, so that the middle portion of each centrum is hidden. From the 175th vertebra to the end of the series there are no longer distinct neural and haemal arches, but simply an irregular vertical plate of cartilage, in which the last eight (?) vertebral bodies are imbedded. An examination of this region in the fcetal specimen (E) shows (Plate VIII. fig. 28) that these are all perfectly formed centra except the last, which is a somewhat irregular mass of bone, and appears to m e to be a demi-vertebra 2, i. e. to represent the anterior half of a centrum formed in the posterior moiety of the last meso-blastic somite. In the skeletons of A and B the neural and haemal arches are entirely uncalcified ; in the large specimen O there are small calcific patches on the anterior neural arches only; from about the 4th or 5th vertebra backwards the only calcifications are those of the centra. b. The Skull.-The cranium is described in detail by Haswell3, who gives figures of it from above and from the right side, which are however, too small to show certain important details, such as the nerve-foramina. For this reason, and because of the desirability 1 T. J. Parker, "On the Skeleton of Eegalecus argenteus," Trans. Zool. vol. xii. p. 24, note. 2 Cf. " Skeleton of Eegalecus;' p. 22. 3 Op. cit., Journ. Linn. Soc. N. S. W . vol. ix. p. 15. |