ECHINODEEMA — BEITTLE-STAES. 
71 
with its coiled arms, a small Hcmigbjplia from the Muschel- 
kalk, and a few other specimens. 
Of Jurassic Ophiuroids, in the British Lias, the so- 
called Starfish bed of Pliensbachian age, exposed between 
Charmouth and Bridport, has yielded OpModerma Egertoni 
and other well-preserved brittle-stars. Interesting forms 
have lately been obtained from the Corallian Calcareous gi’it 
of Yorkshire. The Foreign Jurassic series consists mainly 
of some elegant little species from the Kimmeridgian litho- 
graphic stone of Solenhofen, belonging to the genera Geocoma 
and OphiuTcUa. 
Another Geocoma comes from rather similar rocks of 
Cretaceous a<>e in the Lebanon. From the English Chalk 
there are Ophioglypha scrrata and other species I’ecently 
described by Mr. Spencer in the monograph referred to above. 
By this time, it will be noticed, the genera have quite a 
modern aspect. 
Ophioglypha 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 
Cainozoic rocks, and to the continuous change in structure 
during geological time, the fossil Sea-urchins, or Echinoids, 
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, each 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 is 
Gallery 
VTII. 
Table-case 
29. 
Wall-case 
17 A. 
Wall-case 
17a. 
Table-case 
29. 
Table-cases 
29, 28, 27. 
Wall-cases 
17A, 16, 16. 
