THOLIASTERELLA. 169 



positions. These fragments are built up nearly exclusively of the umbrella 

 spicules, and they thus clearly show that we are dealing with a genus of Sponges 

 fundamentally distinct in the character of their component spicules from those of 

 Holasterclla and Aster actinella. One distinguishing feature of the genus is the 

 continuous character of the dermal layer, the rays of the larger spicules in it 

 being generally fused and amalgamated together. On the other hand, the number 

 and complete forms of the detached spicules present in the same deposits with the 

 portions of the dermal layer, indicate that the majority of the spicules forming the 

 interior skeleton of the Sponge were only held together by the soft animal struc- 

 tures of the organism. 



The spicules of this genus do not show any derivation from normal hexac- 

 tinellid spicules. The number of the transverse or horizontal rays varies from five 

 to nine, and as these are apparently equally developed, and radiate at equal angles 

 from the centre, there is no room for supposing that they result from a modifica- 

 tion of the four transverse rays of the typical hexactinellid spicule. On the other 

 hand, the inconstancy in the number of the rays, as well as the general characters 

 of the spicules, and their union together (in the dermal layer) separate them very 

 decidedly from the spicules of Astrceospongia. 



The spicules of this genus are siliceous and in a similar condition of preserva- 

 tion as those of Hyalostelia and other siliceous Sponges in the same deposit. 



Four species can be recognised in the Carboniferous strata of Ayrshire, and 

 one of these also occurs in Germany. The genus does not apparently pass higher 

 than the Carboniferous. 



45. Tholiastekella Youngi, Hinde. Plate VII, figs. 2, 2 a — 2/. 



1883. Holastebella Youngi, Hinde. Cat. Toss. Sponges, p. 152, pi. xxxii, 



figs. 3—3 d. 



1877. Stellate spicules, Young and Young. Ann. and Mag. Nat. Hist., ser. 4, 



vol. xx, p. 420, pi. xiv, figs. 13, 

 19, 24, 27, 29. 



1878. Hyalonema Smithii ? (in part), Carter. Ann. and Mag. Nat. Hist., 



ser. 5, vol. i, p. 133, pi. ix, 

 fig. 10. 

 1880. Hyalostelia Smithi, Steinmann. ZeitscLr. d. deutsch. geol. Gresell., 



p. 395, pi. xix, fig. 5. 



The skeletal-spicules included in this species have stout, straight, conical shafts, 

 and from five to nine transverse rays, which may be either horizontal or slightly 

 incurved. There is a well-marked central disc to the spicules, and its upper 



