FOSSILS OF UNCERTAIN AFFINITIES. 297 



inter-radial spaces. The respiratory organs or " hydrospires," 

 lastly, are reduced to ten fissures situated on the sides of, 

 and parallel with, the brachial grooves. 



FOSSILS OF UNCERTAIN AFFINITIES. 



We may very briefly consider here four genera which 

 have been at one time or other regarded as belonging to the 

 Cystoidea, but which probably have really affinities of a 

 very different nature, though their exact zoological position 

 is still open to doubt. The genera in question are Pasceolus, 

 Sphcerospongia, Nidulites, and Cyclocrinus, all of them found 

 in the Silurian or Devonian rocks, all obviously related to 

 one another in form and general structure, and all as yet 

 partially understood. If they are Cystideans, they can only 

 be regarded as a peculiar and very aberrant group of this 

 order: but it would seem more likely that they will find 

 their true place in the vicinity of Receptaculites and Ischadites. 



The genus Pasceolus (fig. 176, a and I) was created by 

 Mr Billings for the reception of some curious Silurian fossils 

 of an ovate or globular form, having an integument com- 

 posed of hexagonal or pentagonal plates, closely united with 

 one another, apparently deeply concave towards the interior, 

 and furnished with a lateral aperture, which is said to be 

 surrounded by six plates differing in form from the rest. 

 A peduncle is also supposed to have been present. Mr 

 Billings first supposed that Pasceolus might possibly belong- 

 to the Tunicata, but he subsequently came to the conclusion 

 that its zoological position was entirely uncertain ; while 

 Messrs Verrill and Niles seem to regard it as a Cystidean. 



The genus Sphcerospongia is in a hardly more satisfactory 

 position. It was founded originally by Salter for fossils 

 such as the Sphceronites tesselatus of the Devonian, believed 

 generally to be of a Cystidean nature, and in general struc- 

 ture nearly resembling Pasceolus. The body (fig. 176, c) is 

 ovate or pyriform, hollow within, and covered by hexagonal 

 or pentagonal plates (fig. 176, d), but there is no evidence 

 of any aperture in the test, nor do we know anything as to 



