ii2 OUR COMMON BRITISH FOSSILS. 



those of Crustaceas, are so alike that it is difficult to 

 tell one from another. All commence life as free- 

 swimming, worm-like larvae. 



The relationship between the Crinoids (or En- 

 crinites) and the other leading members of Echinoder- 

 mata is undoubtedly bound still more nearly together 

 by the intercalence of several extinct groups. Thus 

 the two extinct orders, Cystidea and Blastoidea, were 

 in some degree intermediate between Encrinites and 

 sea-urchins, in a manner that we have now no living 

 examples of. The Tortoise-Encrinites of the Chalk 

 (Marsupites\ and the Saccosoma of the Oolite (most 

 probably allied to the Feather-stars), are deeply 

 interesting, inasmuch as they are stalkless fossil 

 Encrinites. 



If we consider the body and arms of an Encrinite, 

 of any species, as a kind of star-fish attached to a 

 jointed stalk, then the base of the Encrinite's body is 

 called the " pelvis ; " the mouth is uppermost, sur- 

 rounded by the feathered arms a position just the 

 reverse of that which would be assumed by a star- 

 fish, for the latter, in crawling over the sea-floor, 

 has the mouth downwards. Both mouth and anus 

 are usually present on the upper surface of the body 

 of a Crinoid, the anus often terminating in a nipple- 

 shaped protuberance. In the most ancient crinoids 

 there seems to have been a difference from the 

 structure seen in their living representatives. If we 

 carefully examine the arms of recent Crinoids, we see 



