112 ECHINODERMA. ASTEROIDEA 



have the ambulacral ossicles alternating on either side of 

 the ambulacral groove. The earliest star-fishes are found 

 in the Tremadoc Beds (Upper Cambrian). The genus 

 Palceaster appears in the Bala Beds. The group is more 

 abundant in the Silurian, where Palceaster, Palwasterina, 

 U raster ella y Lepidaster, and Palceocoma occur. P alabaster, 

 Aspidosoma and many others are found in the Devonian. 

 Jurassic forms have been referred to the genera Solaster, 

 Astrospecten, and Plumaster. Calliderma, Metopaster, 

 Pentagonaster, and Mitraster occur in the Upper Cre- 

 taceous. In the Cainozoic rocks of England star-fishes are 

 rare. 



CLASS II. OPHIUROIDEA 



In the Ophiuroids or brittle-stars, the body consists 

 of a disc and arms. The arms are generally five in 

 number, usually simple, but in some cases branched ; they 

 are much smaller than in the star-fishes and are sharply 

 marked off from the disc and do not contain prolongations 

 of the generative and digestive systems. Usually they 

 are long, cylindrical, and very flexible, serving for locomo- 

 tion by means of movements which take place chiefly in 

 a horizontal direction ; the ambulacral groove is not open 

 to the exterior except, apparently, in a few Palaeozoic forms. 

 The arrangement of the nervous and the water-vascular 

 systems is similar to that found in the Asteroidea, but the 

 tube-feet are not provided with sucking-discs and there 

 are no ampullae. The madreporic plate is on the oral 

 surface. There is no anus, and pedicellariae are nearly 

 always absent. 



Generally there is a well-developed skeleton. In a 



