xvn CLADOPHIURAE FOSSIL OPHIUROIDEA 50 I 



The order is divided into three families, two of which are repre- 

 sented in British waters. 



Fam. 1. Astroschemidae. Arms unbranched. Astronyx is 

 comparatively common in the sea-lochs of Scotland. There are 

 a series of pad-like ridges on the arms, representing the side- 

 plates and bearing the spines. Astroschema. 



Fam. 2. Trichasteridae. Arms forked only at the distal ends. 

 Trichaster, Astrocnida. 



Fam. 3. Euryalidae. Arms forked to their bases. Gorgo- 

 nocephalus is occasionally taken in deep water off the north coast 

 of Scotland. In it the arms repeatedly fork, so that a regular 

 crown of interlacing arms is formed. The animal obviously 

 clings to external objects with these, for it is often taken in 

 fishermen's nets with its arms coiled around the meshes. The 

 genital bursae are said to be represented by slits which open 

 directly into the coelom. (Lyman describes the coelom as divided 

 into ten compartments by radiating septa ; it is possible even 

 probable that these are really the bursae.) An allied species 

 is common in the Bay of Fundy, being found in compara- 

 tively shallow water. Astrophyton (Fig. 222) is closely allied to 

 Gorgonocephalus, differing only in trifling points. It is doubtful 

 whether the separation of these two genera is justified. 



Fossil Ophiuroidea. The Ophiuroidea are rather sparsely 

 represented among fossils, but in the Silurian and Devonian a 

 series of very interesting forms occur which are intermediate in 

 character between Starfish and Brittle Stars, and which were 

 therefore in all probability closely allied to the common ancestors 

 of modern Ophiuroids and Asteroids. Jaekel ? has recently added 

 largely to our knowledge of these primitive forms, and has 

 described a number of new genera. Thus Eophiura from the 

 Lower Silurian has an open ambulacral groove, and the vertebrae 

 are represented by an alternating series of quadrate ossicles, each 

 deeply grooved on its under surface for the reception of the 

 tentacle, which was not yet (as in modern forms) enclosed in the 

 vertebra. The lateral or adambulacral plates extended horizontally 

 outwards, and each bore a series of spines at its outer edge. 



A remarkable fact is that where the halves of the vertebrae 

 (i.e. the ambulacral ossicles) diverge in order to form the mouth- 



1 " Asteriden und Ophiuriden aus dem Silur Bohmens," Zeitschr. der deutschen 

 geol. Ges. lv. 1903, pp. 106-113 (Protokolle). 



