Page:Memoirs of the Geological Survey of Great Britain, Volume 1.djvu/430

Rh 66. Nucula proxima, Gould?

, fossil. Among the drift fossils collected by Captain James, in Wexford, is a Nucula, which comes nearer to the above species of Gould, as figured in the 'Invertebrata of Massachusetts' (fig. 63), than to any species with which I am acquainted. The Nucula nitida of Sowerby may, as suggested by Dr. Gould, be the same species; but, if I am right as to the form usually so called, it is only a variety of N. nucleus.

, recent. Seas of Massachusetts. If a deep-sea British form, taken by Mr. W. Thompson in the North of Ireland, and by Mr. MacAndrew on the West of Scotland, be identical, this will add another to the list of species common to Europe and America.

67. Nucula tenuis. (Arca sp.), Montagu.

, Nucula tenera, S. Wood.

, fossil. In the Clyde beds, and in Ireland. In the mammaliferous crag of South Wold.

, living. In the British (chiefly North) Scandinavian and Arctic Seas. Seas of Greenland and Boreal America.

Note. Appears in the red crag.

68. Nucula Cobboldiæ, Sowerby (M. C).

., fossil. In the mammaliferous crag of Bramerton. Bridlington. A fragment found by Captain James in the Wexford beds probably belongs to this species. It is not now known living. It appeared within our area during the Red Crag epoch, and was probably extinguished by the upheaval of the limited area through which it ranged. It is the finest species of its genus.

Leda. This genus was established by Schumacher for the reception of the beaked Nuculæ. It has been lately revived, and more precisely defined by P. C. Möller, in his 'Index Molluscorum Groenlandiæ.' In that treatise the author groups the forms of Nuculæ inhabiting Greenland under three genera, Leda, Nucula, and Yoldia. Of the first, the Nucula minuta is an example; of the second, Nucula nucleus; and of the third, Nucula arctica. The shell of the first is beaked, and very inæquilateral ; of the second, longitudinally ovate, oblique, and inæquilateral; and of the third, transversely oblong, and usually nearly æquilateral. The essential characters of the animals are enumerated as follows:—

Leda. "Animal tubis, brevibus, tenuibus, rectis præditum; pede longo, tenui, flexili; pallio toto aperto, marginibus simplicibus."

Nucula. "Animal sine tubis exsertilibus, pede brevi, crasso; pallii parte solum inferiore aperta."

Yoldia. "Animal tubis longis curvatis instructum; pede magno, valido; pallio toto aperto, marginibus posticè ciliatis."

I have never seen the animal of a Yoldia; but, having compared those of Nucula and Leda, and the latter with the description given by Möller of Yoldia, I am inclined to restrict the division of the genus Nucula into two generic groups instead of three, and to unite Yoldia with Leda, under the latter name. The animal of Leda minuta, as will be seen by the following sketch,