ECHINODERMA — BRITTLE-STARS. 



71 



with its coiled arms, a small Hcmiglyplia from the Muschel- G-allery 

 kalk, and a few other specimens. VIII. 



Of Jurassic Ophiuroids^ in the British Lias, the so- Table-case 

 called Starfish bed of Pliensbachian age, exposed between 

 Charmouth and Bridport, has yielded Ophioderma Egertoni 

 and other well-preserved brittle-stars. Interesting forms 

 have lately been obtained from the Corallian Calcareous grit 

 of Yorkshire. The Foreign Jurassic series consists mainly Wall-case 

 of some elegant little species from the Kimmeridgian litho- ^^a. 

 graphic stone of Solenhofen, belonging to the genera Geocoma 

 and Ophiurclla. 



Another Geocoma comes from rather similar rocks of W"all-case 

 Cretaceous a^e in the Lebanon. From the English Chalk Table^case 

 there are Ophiojlypha serrata and other species ]'ecently 29. 

 described by Mr. Spencer in the monograph referred to above. 

 By this time, it will be noticed, the genera have quite a 

 modern aspect. 



Opliioglyphci Wetherelli, from the London Clay, is the 

 most important of the British Tertiary sand-stars ; there 

 is also an Ophiolepis from Pleistocene deposits of the Clyde 

 basin. 



Class ECHINOIDEA. 



Owing to their abundance, especially in Mesozoic and Table-cases 

 Cainozoic rocks, and to the continuous change in structure -^jf Jg 

 during geological time, the fossil Sea-urchins, or Echinoids, i7a, le, 15. 

 are of great value to the strati graphical geologist and of no 

 less interest to the student of evolution. 



The differences between a sea-urchin and a starfish have 

 sometimes been illustrated by imagining a starfish with short 

 rays, and therefore with a five-sided or globular shape ; then 

 suppose the grooves to grow upwards to the neighbourhood 

 of the anus so that they supplant all the leathery loose- 

 plated skin, except a small area just round the anus ; let 

 this area be surrounded by five plates, eacli pierced by a 

 pore for the passage of the generative products, and one of 

 them also serving as madreporite — then one would have 

 something very like a sea-urchin. But there is an obvious 

 difference : in the starfish the radial water- vessel lies in a 

 groove outside the skeleton ; in the sea-urchin there is no 

 groove, but a series of plates flush with the rest of the test, 

 and the water- vessel lies beneath these — that is, within the 

 skeleton — and the podia pass out through pores between or 

 in those plates. Thus the test of a regular sea-urchin ia 



