118 BRITISH PALEOZOIC SPONGES. 



spicules cannot be recognised. Longitudinal and transverse sections, however, 

 clearly show its affinities with the genus, and so far as can be ascertained from the 

 condition of the specimen it belongs to the present species. 



Distribution. — Ordovician : Girvan, Ayrshire (Prof. H. A. Nicholson) ; Trenton 

 limestones, near Chicago (Dr. W. R. Head). Silurian : Wange, Isle of Gothland 

 (Prof. G. Lindstrom) ; St. Petersburg, Russia ; Perry County, Tennessee (F. 

 Roemer) ; Lower Helderberg Group, Dalhousie, New Brunswick ; Scoharie, New 

 York. Glacial drift : Sadewitz, Lower Silesia (F. Roemer) ; Lyck, East Prussia ; 

 Rombitten, West Prussia (F. Roemer) ; Island of Sylt, Holstein (Haas). 



6. Hyalostelia Smithii, Young and Young sp. Plate I, figs. 4, 4 a. 



1877. Htalonema Smithii, Young and Young (in part). Ann. and Mag. Nat. 



Hist., vol. xx, p. 426, pi. xiv, 

 figs. 1—3, 5—12, 14—17. 



1880. — ? Girvanense, Nicholson and Etheridgejunr. Mon. Silur. Foss. 



Girvan, Fasc. ii, p. 239, pi. xix, 

 figs. 1—1 b. 



The references to this species and its characters will be more fully given in 

 treating of the Carboniferous Sponges ; it is introduced here to include a specimen 

 of elongated spicular rods from Ordovician strata. 



The spicular rods in the only specimen known from this horizon are not united 

 in bundles, but they are detached and distributed irregularly at short distances 

 from each other in the rocky matrix. They are circular in transverse section, 

 with apparently smooth surfaces. Their length and natural terminations are 

 unknown. They vary very considerably in thickness, the slender rods not 

 exceeding '15 mm. in diameter, whilst the stoutest spicules are 1*4 mm. The axial 

 canals are occasionally preserved. 



These spicules were referred by Messrs. Nicholson and Btheridge to a distinct 

 species, principally on account of peculiar transverse bands of varying thickness, 

 which occur at intervals in the spicules and were believed to indicate a distinct 

 structural feature. In sections of the type specimen from which the figures on 

 Plate I are drawn, the spicules exhibit, by polarized light, the optical characters of 

 chalcedonic and crystalline silica, but in the banded intervals the silica has evidently 

 been replaced by some other mineral. The replacement has been effected along 

 minute transverse fissures in the spicules, and the same mineral has likewise been 

 deposited in places on their outer surfaces. It seems clear, therefore, that the 

 bands are not original, but merely secondary structures resulting from fossilization. 



In their relative proportions these spicular rods agree with those forming the 



