This beautiful and highly interesting shell has been generally considered _unique_ among the collections in this country. It was originally in the possession of the late Mr. Jennings, and, I am informed by Captain Laskey, was on first being received, in a much finer state. Mr. Jennings had it cleaned, and in so doing many of the delicate transverse striae were partially obliterated, and the sharp ridges on the longitudinal ribs worn down, as indeed was apparent from a drawing Captain L. had made of the shell previous to this unmerciful cleaning. It however still remains a very fine shell, and is now in the cabinet of Mrs. Bolton, of Storr's-hall, to whom I am obliged for the opportunity of now publishing it. The figure and specific character will sufficiently point out its distinctions. The body-whorl is smooth, but strongly granulated at the base; the spire delicately striated between the ribs; the two last whorls before the apex are close, thick, and somewhat papillary; the apex itself small and sharp. The upper part of the inner lip has some faint obsolete teeth, but the base has three very strong ones. It connects in the most beautiful manner the two genera of _Mitra_ and _Voluta_. Its country is unknown. * * * * * Pl. 55 [Illustration] SOLEN ambiguus. _Ambiguous Solen._ * * * * * GENERIC CHARACTER. _Testa bivalvis, aequivalvis, transversissime elongata, utroque latere hians. Dentes cardinales parvi, fragiles, numero variabiles, raro divaricati. Ligamentum externum; animal ad extremitatem anteriorem pede subcylindraceo; ad posteriorem siphone brevi duos alteros conjunctos continente._ Lamarck. Typus Genericus _Solen Vagina_ Pennant. Shell bivalve, equivalve, very transversely elongated, open at both ends. Cardinal teeth small, fragile, variable in number, and rarely divaricated. Ligament external. Animal with a sub-cylindrical foot at the anterior end, and at the other a short tube containing two others united together. _Lamarck._ Generic Type _Solen Vagina_ Pennant. * * * * * SPECIFIC CHARACTER. _S. testa lineari, crassa, recta, pallida, obscure radiata; cardinibus unidentatis, margine anteriore sub-approximantibus._ Shell linear, strong, straight, pale, obscurely radiated. Cardinal teeth one in each valve, placed near the anterior extremity. Solen ambiguus. _Lam. Syst. vol._ iii. _p._ 452. _no._ 7. * * * * *