_Generative System._--The penis is remarkably acuminated; the vesiculæ seminales are unusually small, and enter only for a short distance into the prosoma; the testes are large. The ovarian tubes are of large diameter; the ova are nearly spherical and large, namely, 9/400ths of an inch in diameter; they are not numerous, and lie in single layers in the two lamellæ. The ovigerous fræna are well developed, and lie under the scuta; one I measured was 5/100ths of an inch in length and 2/100ths in width; the margin is obliquely truncated and slightly sinuous. This species breeds late in the autumn, and even in mid-winter; I have examined a specimen from Cornwall with ova containing larvæ, taken on the 26th of October; again, in another specimen from Belfast, sent to me by Mr. Thompson, taken in January, there were ova in the lamellæ, and therefore no doubt impregnated; and on February the 12th I received from Mr. Peach, from Cornwall, specimens so very young that they must have become attached during the first days of the month. _Varieties._--The specimens from near Naples, (which I owe to the kindness of the Rev. F. W. Hope,) are somewhat larger, and differ slightly from those of Britain: they form, I imagine, the _S. Siciliæ_ of Chenu. After carefully examining them internally and externally, I think it is quite impossible to consider them specifically distinct, for although in several specimens, the valves were placed a little further apart from each other,--the upper latera a little more elongated,--the carinal latera rather narrower in their upper half,--the infra-median latera rather more rounded,--and, lastly, in the scuta, the tergal margin extended almost in the same line with the lateral margin; nevertheless in other specimens, I could perceive no difference whatever. It is, however, remarkable that in several full-grown Neapolitan specimens there were no Complemental males, whereas I have never seen a single full-grown British specimen without such being present. In some specimens in the British Museum, without any given locality, I have observed considerable variation in the breadth of the carinal and rostral latera. COMPLEMENTAL MALE. Pl. V, figs. 9-14.