the Crlnoidea, Cjstidca and Blastoidea. 



253 



imbedded in and form a portion of the general covering of the 

 body, or lie npon the surface, or be borne upon free-moving 

 arms. In genera belonging to the same family these relations 

 are constant or nearly so, -but are found to be extremely vari- 

 able when ditferent orders or remotely allied families are 

 compared. 



While preparing my Decade No. 3, I investigated this sub- 

 ject, and satisiied myself that in at least a large proportion of 

 the palaeozoic Crinoids the mouth was disconnected altogether 

 from the radial system. A great many species might be re- 

 ferred to in which we can see both the centre from which the 

 ambulacra proceed, and the mouth, and at the same time see 

 that they are not in the same i)lace. A long train of reason- 

 ing is not necessary, only simple inspection. It will be quite 

 sufficient to notice a few of these species to prove that the 

 rule laid down by Prof. Wyville Thomson is not a general 

 rule. 



Fiff. 1. 



Fig. 2. 



Fig. 1. This figure is a diagram of the interior of the vault 

 of a Crinoid which appears to be Batocrinus icosidactylus 

 (Cassiday), a fossil that occurs in the Carboniferous rocks of 

 Kentucky. It was sent to me by Mr. 8. S. Lyon, of Jefferson- 

 ville, Indiana, several years ago. The test is in a beautiful 

 state of preservation, and perfectly empty, so that all of the 

 markings on the inner surface can be distinctly seen. There 

 are twenty-one arms arranged in five groups («), and the same 

 number of ambulacral openings (^^), each just large enough to 

 admit of tlie entrance of a slender pin. The mouth {mv) is 

 nearly ccuitral ; and close to it, on the posterior side, there is a 

 small rudely pentagonal space (c) with no markings except 

 several small tubercles. The grooves are scarcely at all im- 

 pressed ; and, indeed, I think they are never so in any Crinoid, 

 except in those which have a thick test. In this specimen 

 their course is clearly indicated by the remains of the thin 

 partitions which either separated them or to which the vessels 

 were attached. They do not run directly toward the mouth, as 



