LYMNORELLA MAMILLOSA. 235 
There is considerable variation in the characters of the sponges included in 
this genus. Some massive forms consist of concentric layers of fibrous skeleton 
with only occasional traces of canals and a few slight depressions of the surface 
to mark where oscules may have existed, but no further indications of individual 
spongites are distinguishable in the common stock ; whilst in other stocks, conical 
spongites, each with a tubular cloaca and an open oscule at its summit, project 
prominently from the mass. These latter correspond with the type-forms of the 
genus ; for those in which canals and oscules are absent or rarely developed I had 
proposed the genus Inobolia (Cat. Foss. Sponges, Brit. Mus., p. 184), but from 
the study of a large series of specimens it is evident that this genus cannot be 
maintained independent of Lymnorella, since gradational forms occur between the 
extremes of the series. 
The microscopic characters of the skeleton fibres in the sponges of this genus 
from the Inferior Oolite of this country, and also in some of the examples from 
the Calcaire a Polypiers, near Caen, exhibit some peculiar features, which may, 
however, result from the fossilisation. In thin sections the fibres show three- 
rayed spicules in the central portion, disposed so that the rays slightly overlap 
each other; they are white by reflected light, but dark granular by transmitted 
light (Pl. XVIII, figs. 2c, 3c, 6). These axial spicules are enclosed by a layer of 
calcite of a lighter tint, which appears to consist of minute crystalline fibres. 
This outer layer sometimes contains traces of filiform spicules, but as a rule 
nothing beyond its crystalline structure is present. It is very distinctly marked 
off from the infilling calcitic or sedimentary matrix (Pl. XVIII, fig. 34), showing 
that it is derived from the original skeleton fibre; and it appears to be probably 
due to a secondary change which has obliterated the smaller spicules in the marginal 
portion of the fibres, and left the larger axial spicules in their present opaque 
granular condition. A somewhat similar condition of the skeleton fibres also occurs 
in Peronidella tenuis (Pl. XIV, fig. 2a). Inone species, Lymnorella micula, from a 
different horizon and locality, the fibres retain their normal characters (Pl. X VIII, 
fio. 5b). 
43. LYMNORELLA MAMILLOSA, Lame. Plate XVIII, figs. 2—2 c. 
1821. Lymnorra Mamtttosa, Lam. Exp. méthod., p. 77, pl. Ixxix, figs. 2—4. 
= = Blainville. Man. d’Actinologie, p. 541. 
1837. Mammrttopora mamittosa, Bronn. Leth. geogn., vol. i, p. 236. 
1878. Lymyorrea MamMiItiaRrts, Zittel. Studien, Abth. iii, p. 128. 
1883. — MAMILLOSA, Hinde. Cat. Foss. Sponges, p. 184, pl. xxxv, 
figs. 1, La, 16. 
