OCR Text |
Show 1897.] ECHIDNOCEPHALUS, A HALOSAUROID EISH. 271 exhibit traces of a longitudinal ridge or angulation. No other scales are preserved. The fourth and last specimen in the British Museum (no. P. 2114) is preserved on a slab with remains of other fishes. Part of its soft tissues are shown in places as a blackened film, but, like the other specimens, it exhibits no clear indication of scales. The low cranium is observable in broken longitudinal section, wdiile there are imperfect impressions of the characteristic pterygo-quadrate arcade and opercular apparatus. There are also impressions of ten very slender and widely-spaced branchiostegal rays. Immediately behind these occurs the clavicle, but no pectoral fin. The vertebras are well shown, of the form and character already described. The delicate ribs are very short, apparently not reaching more than halfway to the ventral border; and there seem to be long and slender intermuscular bones crushed across the neural arches both in the abdominal and caudal regions. One of the pelvic fins exhibits six rays, all except the foremost divided in the distal half; its support is longer than broad and tapers to a point in front; it is shown in the impression. Six rays are well preserved in the dorsal fin, and there may have been one or two more beyond. The first of these rays is simple and a little shorter than the others ; the second is also simple, but slightly longer and with distant articulations ; the third is the longest ray, while this and the other three are once bifurcated distally. The anal fin is imperfect at its free border, and the end of the tail is wanting. So far as the characters of Echidnocephalus are shown by these specimens, the Cretaceous fish only appears to differ from the Becent Halosaurus in three particulars: no scales are observable in the British Museum fossils except along the sensory canal of the "lateral line"; no pectoral fin is distinguishable; and the number of rays in the dorsal and pelvic fins is less than is usual in the existing genus. The first two of these differences, however, may be due to imperfections in preservation ; and Dr. von der Marck has indeed mentioned l that some specimens exhibit very delicate scales, covering the whole of the trunk. The third point is comparatively insignificant. Other differences may still be discovered in the characters of the facial bones and dentition, which remain unknown ; but, in any case, it will be realized that in all essential features the Halosauroid type of fish is one of great antiquity. E X P L A N A T I O N O F P L A T E XVIII. Figs. 1-3. Echidnocephalus troscheli, W . von der Marck.-Upper Cretaceous (Senonian); Sendenhorst, Westphalia, br., branchiostegal rays ; cr., cranial roof; cl., clavicle ; ecpt., ectopterygoid ; enpt., entopte'rygoid ; /., enlarged scales of " lateral line" ; md., inaudible ; mpt., nietapterv-goid ; op., operculum; pas., parasphenoid; pmx., premaxilla; qii., quadrate; s.op., suboperculum. [The figures are of the natural size, and the original specimens in the British Museum are numbered respectively P. 2111, P. 4481, P. 5949.] 1 Paleontographica, vol. xv. (1868), p. 288; ibid. vol. xxii. (1873), p. 62 and ibid. vol. xxxi. (1885), p. 260. |