Carboniferous of the S.W. of Scotland. 137 
fact, as the following description of a fossil sponge from the 
Lower Limestone of the Carboniferous system of S.W. Scot- 
land, sent to me by Mr. Thomson, affords another instance 
of a Renierid sponge, in form of spicule somewhat like 
Pharetrospongia Strahani, having passed from the siliceous 
into the calcareous state. 
Pulvillus Thomsonii, n. sp. (Pl. X. figs. 1-6.) 
Calcareous fossil. Pulvinate, circular, depressed towards the 
centre on both sides, contracted towards the circumference, 
which is round or angular, elevated between (Pl. X. fig. 1). 
Surface uniformly granular, interrupted by a central circular 
excavation on each side, one of which is much larger than 
the other (fig. 1,a), and the smallest filled with a stem-like 
fragment (fig. 3,a,6). Internal structure granular through- 
out (fig. 2); granules subround, variable in size, below 1-8th 
inch in diameter, composed of crystalline calcite, which in the 
thin vertical section is semitransparent, of a light brown colour 
and sometimes white (fig. 2, dd) ; imbedded in dark material 
composed of a heterogeneous mixture of minute particles of 
sand and organic fragments, often giving place to white semi- 
crystalline calcite (fig. 2, e,e, e) ; the whole, in a vertical or 
horizontal section, presenting the appearance of a granular, 
minutely veined conglomeration, wherein the veins, especially 
towards the large excavation (fig.2,a), are much wider than the 
rest, into which they afterwards appear to become subdivided. 
Granules largest on the side which is most excavated (fig. 2, a), 
and surrounded generally by a thin proper layer, which may 
be of a dark lead- or ochraceous yellow colour, according to 
the specimen; presenting, in a vertical section, bundles of 
smooth, slightly curved, acerate, white, opaque or clear trans- 
parent spicules, cut across more or less longitudinally into 
variable lengths by the plane of the section (fig. 4, a, 6), which, 
when passing through the granules horizontally, fails, except 
here and there, to show more than the crystalline calcite. 
Broken ends of the spicules abundant in, and projecting from, 
the surface of the large excavation, where, from their trans- 
parent, crystalline nature, they appear, for the most part, 
in the form of dark, circular, transverse sections of various 
sizes, in the midst of each of which is a punctum representing 
the axial canal (fig. 6,a,4). Spicule smooth, acerate, fusi- 
form, curved, and gradually attenuated to a point at each end; 
variable in size, about 1-25th by 1-600th inch in its largest 
dimensions, the only perfect one seen being smaller, viz. 
1-45th by 1-900th inch in its largest dimensions (fig. 5). 
Size of largest specimen of entire fossil about 5 inches in 
Ann. & Mag. N. Hist. Ser. 5, Vol. i. 10 
