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 say. 



