of the Genus Agelacrinites. 



161 



sessile — is attached by its under surface to shells and other 

 foreign bodies ; and hence the mouth cannot be situated there. 

 Several examples, it is quite true, have been met with attached 

 in this manner to Brachiopod shells ; but this is by no means a 

 general condition of occurrence, and, rightly considered, is no 

 proof of an original permanent attachment. It is just as excep- 

 tional a mode of occurrence, indeed, as that from which Vanuxem 

 derived the name of the genus. 



This suggestion as to the true position of the mouth cannot, 

 of course, be satisfactorily adopted until confirmed by the ex- 

 amination of more perfect specimens than those hitherto dis- 

 covered, or until the proper functions of the pyramidal orifice, 

 in this genus and in the Cystideans, are clearly ascertained. 

 But, under any view, it seems obvious that, without a forced 

 collocation, these peculiar forms cannot be placed in any existing- 

 group. In the present restricted state of our knowledge at least, 

 they must form a group apart. Mr. Billings (Decade III. of 

 f Canadian Organic Remains/ under description of Agelacrinites 

 Dicksoni) appears inclined to regard them as constituting a sub- 

 order of Star-fishes ; and he proposes to arrange them in this 

 connexion under the term of Edrio aster idee. This name seems 

 objectionable, however, on two grounds : first, because the sup- 

 posed sessile (i. e. parasitic) condition of Agelacrinites is by no 

 means proved ; and secondly, because the relations of the genus 

 to the Star -fishes — in so close a way, at least, as the name would 

 imply — is not yet established. For these reasons I would sug- 

 gest the term Thyroidea, in allusion to the valved aperture, as the 

 name of the special group or order framed for the reception of 

 these forms. The following scheme will then represent the pro- 

 bable relations of the various leading groups belonging to the 

 Echinodermata generally :— 



In the group Thyroidea we have, at present, but one family — 

 that of the Agelacrinitida, comprising, probably, but one known 



