132 BRITISH PALEOZOIC SPONGES. 



The general outline of these Sponges is that of an elongated ellipse ; the sides 

 are occasionally straight, but usually slightly curved. The basal portion is in 

 most cases narrower than the summit, it is evenly rounded, and there are no traces 

 of stem or of fibres by which it might have been attached. The basal and lateral 

 margins are clearly defined ; the summit portion is less frequently exposed, but it 

 appears to have been evenly rounded. There is considerable variation in the size 

 of different specimens; a small example measures 17 mm. in length by 8 mm. in 

 width, whilst a large form is 60 mm. in height by 23 mm. in width. In the 

 present compressed condition the thickness of the Sponge is inconsiderable, about 

 1 mm. in the upper portion and 2*5 mm. at the base. 



It is uncertain whether a central cloacal cavity existed, but it is possible that 

 the upper portion of the Sponge may have originally been a hollow sac with thin 

 walls, which are now compressed together. In one specimen I have seen indica- 

 tions of a narrow cavity in the basal portion. 



The conical spicules extended from one-fourth to one-half of the entire length 

 of the Sponge. Judging from casts, their surfaces were smooth and even, and 

 they tapered evenly from the rounded head to the pointed extremity. They varied 

 from 2 to 3 mm. in length, and from *5 to 1 mm. in thickness. At the extreme 

 base of the Sponge these conical spicules pointed directly upwards, whilst on the 

 sides they are disposed obliquely to its axis, their points being directed down- 

 wards and inwards. The four- and five-rayed spicules forming the upper portion 

 of the Sponge are very much smaller than the conical basal spicules, on which 

 they directly rest. They are so closely arranged that, as a rule, only the casts of 

 one spicular axis and the small aperture of a third ray can be seen on the exposed 

 surface of the Sponge, and their real forms can only be ascertained from the casts 

 of the detached individuals, which are fairly abundant in the matrix associated 

 with the conical spicules. The rays of the smaller spicules are straight, they very 

 gradually taper from the central node, and they vary from *5 to 1 mm. in length, 

 and about *12 mm. in thickness. The slender filiform spicules are seldom pre- 

 served, they appear to have been arranged somewhat obliquely to the length of 

 the Sponge, and to have been restricted to its upper portion. 



All the examples of this species hitherto found are preserved as casts in a soft 

 brown, micaceous, somewhat shaly rock. They are extended on the bedding- 

 plane of the rock, sometimes in a uniform direction with respect to each other, 

 and in such numbers as nearly entirely to cover the surface (PI. Ill, fig. 3). 



The type of the species is now in the Museum of the Geological Survey, Jermyn 

 Street, but much better preserved specimens have been discovered since the form 

 was first described, and these are now in the British Museum, and in the Museum 

 of Science and Art, Edinburgh. 



Distribution. — Silurian : Upper-Ludlow strata ; Wetherlawlinn, Pentland Hills, 

 near Edinburgh. 



