1 86 FOSSILS OF DOUBTFUL AFFINITIES. 



same time, the skeleton departs widely in certain of its features 

 (such as the presence of a distinct perforated internal wall) from the 

 Madreporarian type. Upon the whole, therefore, in the present 

 state of our knowledge, it seems best to regard the Archceocyathinoz 

 as a group of uncertain affinities, probably more closely allied to the 

 Madreporaria than to any other division of the animal kingdom. 



II. Pasceolus, Cyclocrinus, and Nidulites. 



We may briefly consider here a series of fossils the true affinities 

 of which are at present absolutely uncertain, and which have no par- 

 ticular claim to be taken up in this connection beyond the fact that 

 they have a superficial resemblance to Receptaadites and Iscnadites, 

 and have therefore been commonly placed alongside of these 

 genera. It has, however, been demonstrated by Hinde that the lat- 

 ter are Hexactinellid Sponges, while it seems certain that the fossils 

 here referred to — viz., Pasceolus and Cyclocrinus — are, at any rate, 

 not referable to the Porifera, though their true position is quite 

 problematical. 



The genus Pasceolus (fig. 73, a and b) was created by Mr Bil- 

 lings for the reception of some curious Ordovician and Silurian 

 fossils of an ovate or globular form, varying in size from the dimen- 

 sions of a hazel-nut to those of an apple. The outer layer of 

 the fossil consists of " small convex elevations, composed of a very 

 thin minutely wrinkled layer, which is sometimes translucent " 

 (Hinde). These surface elevations have the general aspect of being 

 hexagonal or pentagonal plates, but there is not sufficient evidence 

 that actual plates are present, and they may be, and probably are, 

 only definitely limited areas of a common calcareous membrane. 

 The inner, concave sides of these polygonal areas are directed in- 

 wards, towards the central cavity of the fossil (the cup-like plates of 

 Cyclocrinus being turned outwards); and in well-preserved specimens 

 they exhibit a minutely porous aspect. There is some evidence 

 that the organism was provided with a peduncle of attachment, and 

 in some species a lateral aperture has been described as present. 

 By Mr Billings, it was at first supposed that Pasceolus might belong 

 to the Tunicata, but this view was subsequently abandoned by him. 

 By later palaeontologists the genus has been doubtfully compared 

 with Receptaadites or with the Cystideans, but its zoological affinities 

 must in the meanwhile be regarded as entirely uncertain. 



The genus Cyclocrinus was originally founded by Eichwald to in- 

 clude certain ovate or spherical fossils from the Ordovician rocks of 

 Esthonia, which appear to have been free, and which have a thin, 

 plated, external layer enclosing a large central cavity (fig. 73, e-h). 

 The outer integument is calcareous, and presents externally the 



