156 SILICEOUS SPONGES. 



peculiar arrangement and combination with the relatively large conical spicules of 

 the basal portion present characters so entirely different from those of any other 

 fossil siliceous sponge, that I am unable to assign it to a definite position in this 

 order. 



My study of this species has been greatly facilitated by the loan of specimens from 

 Prof. Nicholson, Dr. Traquair, and from the collection of the Geological Survey of 

 Scotland, through the kind permission of Prof. A. Geikie, Director-General of the 

 Survey. 



Distribution. Silurian strata of Upper Ludlow age: Pentland Hills, near Edin- 

 burgh. 



Genus MORTIERA, De Koninck, 1842. 



MoKTiERA VERTEBKALis, De Koninck. 



1842. Mortiera vertebralis, De Kon. Anim. foss. carbon. Belg. p. 12, t. B. f. 3. 



1846. Mortiera vertebralis, Mich. Icon. Zooph. p. 253, t. 59. f. 1 a, 1 b. 



1852. Mortiera vertebralis, Milne-Edw. & Haime, Brit. Foss. Corals, p. 209. 



1872. Mortiera vertebralis, De Koninck, Nouv. Rech. sur les Anim. foss. carbonif. Belg. 



p. 163, 1. 15. f. 9, 9 a. 

 1880. Mortiera vertebralis, P. A. Roemer, Leth. Geogn. 1 Th. p. 322, t. 39. f. 11 a, 11 6. 



Sponges detached, growing in the form of short cylinders with biconcave ends, and 

 presenting a superficial resemblance to the centrum of a fish vertebra. The speci- 

 mens vary from 8 to 36 mm. in height and from 26 to o5 mm. in diameter. The 

 wall of the sponge is composed of numerous vertical radiating lamellae, which 

 extend to the centre in the same manner as the septa of a coral. The specimens are 

 now calcareous and the minute structure has been obliterated, but traces of spicules, 

 apparently of a Tetracladine character, can be seen in transverse sections. In one 

 specimen, which has been less exposed to weathering influences, the exterior surface 

 is formed of a layer of elongate acerate spicules, about 4 mm. in length, disposed 

 parallel to the axis of the sponge. 



This species was regarded by De Koninck as a coral allied to Cyclolites ; F. Roemer 

 placed it with the sponges from the presence of fragmentary spicules after treating 

 specimens with acid. There is no doubt that it is a true sponge, and it probably 

 belongs to the Lithistidse, though its proper position cannot be determined until the 

 characters of the interior spicules are ascertained. The septate disposition of the 

 wall is not peculiar to this species, for a similar arrangement of the wall-tissues also 

 occurs in the Lithistid sponges Cnemidiastrum stellatuni, Goldf. sp., Corallidium 

 diceratinum, Quenst. sp., and also in sponges of the genus Seliscothon, Zitt. 

 Distribution. Carboniferous Limestone : Tournay, Belgium. 



