. The Cambridge natural history. Zoology. 476 ECHINODERMATA ASTEROIDEA CHAP. XVI Pentaceros appears. In the older rocks occur a number of forms of different character from any now existing. Of these Aspidosoma (Fig. 206), with short lancet-shaped arms sharply distinguished from the disc and continued along' its under surface, seems to be intermediate between Asteroidea and Ophiuroidea. The skeleton of the arm is composed of alternating ambulacral ossicles bordered by adambulacral ossicles, which are at the same time marginals and sharply distinguished from the marginals forming the edge of the
. The Cambridge natural history. Zoology. 476 ECHINODERMATA ASTEROIDEA CHAP. XVI Pentaceros appears. In the older rocks occur a number of forms of different character from any now existing. Of these Aspidosoma (Fig. 206), with short lancet-shaped arms sharply distinguished from the disc and continued along' its under surface, seems to be intermediate between Asteroidea and Ophiuroidea. The skeleton of the arm is composed of alternating ambulacral ossicles bordered by adambulacral ossicles, which are at the same time marginals and sharply distinguished from the marginals forming the edge of the disc. Palaeaster, on the other hand, is a true Asteroid; there are marginals distinct from the adambulacrals, but the disc is reduced to its smallest dimensions, there being only one plate. Fig. 206.—Three views of Aspidosoma, a fossil Asteroid. A, oral view ; B, aboral view of one arm ; C, enlarged view of a portion of the ambulacral groove, adainb, Adambulacral plate ; mnb, ambulacral plate ; marg, marginal plate ; pody aperture for extension of tube-foot. on the ventral side of each interradius. There are a number of genera (Palaeocoma, for instance) with a large disc and very short arms and very shallow ambulacral grooves ; all have alternating ambulacral plates. Some genera appear to have had the madre- porite on the ventral surface of an interradius. On the other hand, in the Devonian occurs Xenaster, which was a fairly normal Asteroid, with pavement-like marginals, deep ambulacral grooves, and broad arms. Thus it will be seen that already in Jurassic times the three orders, Forcipulata, Paxillosa, and Spinulosa were differentiated from each other, but how these are related to the older Palaeozoic forms it is at present impossible to Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Harmer, S. F
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectzoology, bookyear1895
