1869.] 



BILLINGS — STRUCTURE OP CRINOIDS, ETC. 



279 



internal or external, and associated with either the mouth, the 

 anus, or with neither. The aiubulacral skeleton may be imbed- 

 ded into and form a portion of the general covering of the body, 

 or lie upon the surface, or 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 variable when different 

 orders, or when remotely allied families are compared. 



While preparing my Decade No. 3, I investigated this subject, 

 and satisfied myself that in, at least, a large proportion of the 

 paleozoic Crinoids the mouth was disconnected altogether i'rom 

 the radial system. A great many species might be referred to in 

 which we can see both the centre, from which the ambulacra pro- 

 ceed, and the mouth; and at the same time see that they are not in 

 the same place. A long train of reasoning is not necesisary- 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 

 Thompson is not a general rule. 



1. 2. 3. 



p 7.- 7/i ?■ P 



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

 of a Crinoid, which appears to be Batocrinus icasodactylus 

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

 Kentucky. It was sent to me by Mr. S. 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 (a,) and the same number of 

 ambulacral openings (jp,) each just large enough to admit of the 

 entrance of a slender pin. The mouth (mv) is nearly central, 

 and close to it, on tlio posterior side, there is a small rudely pen- 

 tagonal space (c) with no markings, except several small tubercles. 

 The grooves are scarcely at all impressed, and, indeed, I think 

 they never are so in any Crinoid, except in those which have a 

 thick test. In this specimen their course is clearly indicated by 



