112 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 Crinolds (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 

 {Marsnpites\ and the Saccosoitia 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 



