Rh. Etheridge, jun.— Carboniferous Tubicolar Annelida. 259 
but concealed by the last volution, attached by one side, and ex- 
panding towards the aperture; umbilicus open and unconcealed ; 
back broad; surface plain or very finely wrinkled across; aperture 
round, and somewhat produced ; gregarious in habit. 
Obs.—S. ambiguus was described by the Rev. Dr. Fleming in his 
paper on the British Testaceous Annelides, and appears to have been 
almost entirely overlooked by subsequent writers. It was placed by 
Dr. Fleming in the second division of his arrangement of the species 
in the genus Spirorbis, or in those with the “shell destitute of 
longitudinal ridges.” This species is decidedly gregarious, living 
in small clusters together, and is almost always found on the shells 
of Mollusca. The surface is plain, with the exception of a few 
accretion striz, which are usually more marked round the margin of 
the umbilicus. 
In the marine limestones and associated strata of the Carboniferous 
period, S. ambiguus appears to take the place of the burrowing 
form S. (Microconchus) pusillus, so essentially characteristic of the 
more brackish-water deposits of the same epoch, especially the 
Calciferous Sandstone Series of Scotland; but I have never seen 
an individual of S. ambiguus from the limestone series with this 
burrowing tendency. 
I can see little or no difference between the specimens of 
S. ambiguus I have examined and the figures of M‘Coy’s S. globosus,}! 
and I strongly suspect that when a comparison of specimens can be 
made, the one will be found to be indistinguishable from the other. 
They are both dextral and globose; both have a broad back for the 
size of the tube; both are to all intents and purposes smooth, and 
the volutions are concealed by the last whorl, if more than one, or 
one and a halfexists. The only apparent difference between the two 
is in the umbilicus, which, in S. ambiguus, certainly cannot be said to 
be small. 
Mr. R. Howse? quotes S. globosus from the Permian of Humbleton 
Hill, so, if identical with the present species, the upward range of the 
latter will be much increased. I am indebted to my friend Prof. 
F. von Roemer, of Breslau, for specimens of Spirorbis omphalodes, 
Goldf., from the Hifel, and I really can hardly distinguish between 
this and well-grown examples of S. ambiguus, so much are they 
alike; the latter is perhaps a little more striated. If identical, I can 
hardly indicate which name has priority. Fleming’s name was 
given in 1828, that of Goldfuss between 1826 and 18388. 
In some specimens of S. ambiguus, whether from accident or not, 
it is difficult to say, the sides of the shell become compressed and 
concave near the periphery, giving to the latter the appearance of a 
bordering keel. This is usually seen in attached specimens and is 
probably accidental. 
The unnamed Spirorbis, figured by Mr. J. de C. Sowerby * from 
1 Synop. Carb. Foss. Ireland, p. 169, t. 4, f. 10. 
2 “ Catalogue,’’ etc., Trans. ‘l'yneside Nat. Field Club, 1848, i. p. 258. 
3 Trans. Geol. Soc. 1840, 2nd ser. v. t. 40, f. le. 
