HYALOSTELIA. Ill 



1873. Pyeitonema fasciculus, Salter. Cat. Cambrian and Silur. Fobs. Cam- 

 bridge, p. 30. 



1881. Eophtton ? explahatum, Nathorst. Om spar af nagra evertebrade djur, 



&c, Kong. Svenska vetensk. Akad. 

 Handl., Bd. 18, No. 7, p. 46. 



1881. — — Sicks. Quart. Journ. Geol. Soc, vol. xxxvii, 



p. 490. 



1883. Hyalostelia fasciculus, Hinde. Cat. Foss. Sponges, p. 151. 



1886. — — — Geol. Mag., dec. iii, vol. iii, p. 337, 



fig. 1. 



No hexactinellid body-spicules are as jet known in connection with this species, 

 which is founded exclusively upon fragments of the bundles of spicular rods forming 

 the anchoring appendages of the Sponge. In some examples the bundles occur as 

 narrow, nearly straight bands of indefinite length ; the longest specimen known is 

 140 mm. in length, from 5 to 6 mm. in width, and with a thickness varying from 

 *5 to 2 mm. ; in others they resemble stout ropes, from 20 to 25 mm. in thickness. 

 The individual rods composing these bundles are, for the most part, in close contact 

 and parallel with each other, and there is no apparent twist in their course. Their 

 axial canals are but rarely preserved, and the natural termination of the spicules 

 is unknown. They are nearly circular in transverse section, and vary from "15 to 

 •7 mm. in thickness. The surface of some of these spicular rods is quite smooth, 

 whilst in others there is a minute projecting frill, disposed in an annular or spiral 

 form, so that the spicule appears to be covered with transverse, slightly wrinkled 

 striae. The spicular rods also occur detached and scattered through the rock, 

 crossing each other in various directions. 



This species was founded by Prof. M'Coy on a fragmentary band of spicules 

 embedded in dark limestone of Llandeilo age. Special mention is made in the 

 description of the irregular transverse plicae on the surface of the spicules, or tubes, 

 as they are termed ; and this structure is clearly shown in the accompanying figure 

 pi. i b, fig. 13 a. In the original specimen, however, now preserved in the Wood- 

 wardian Museum, Cambridge, the "plicae" are very indistinct, and they can 

 scarcely be distinguished from fractures in the spicules. In other specimens from 

 Llandeilo rocks, the transverse frills are very prominent, and they form one of the 

 distinguishing characters of the species. In some bundles, nearly all the spicules 

 are frilled, but in others, only one in ten, or one in twenty are thus ornamented, 

 whilst the others are quite smooth. In the anchoring rope of the recent Hyalonema, 

 Gray, some of the spicules are likewise furnished with spiral frills, 1 bearing minute 

 spines, thus showing a general correspondence in structure to these Cambrian 

 forms. 



Slight differences exist in the maximum thickness of the spicular rods in 

 different bundles; for, whilst in some the largest spicules do not exceed "5 mm. in 

 1 Carter, 'Ann. and Mag. Nat. Hist.,' ser. 4, vol. xii (1873), p. 372, pi. xiv. 



