Genus Ammodiscus, Reuss.
Ammodiscus tenuis, Brady.
Ammodiscus tenuis, Brady, 1881, Quart. Journ. Micr. Sci., vol. xxi, n.s., p.51. Id., 1884, Rep. Chall., vol. ix, p. 332, pl. xxxviii, figs. 4–6.
The form distinguished as A. tenuis occurs with some frequency in our sounding. It has been previously recorded,* in company with A. incertus, d'Orb., from a neighbouring locality—station 169; and Dr. Brady, indeed, threw out the suggestion that the form was perhaps only a local variety of the better-known and ubiquitous species A. incertus.
Dr. Rhumbler has also remarked about this species,† “Die Scheibe ist dunner, die welcher sie vielleicht die megalosphirische Form darstellt.”
A general examination of our specimens showed the initial portion of the coil in every case to commence with a more or less swollen “protoconch” or (?) megalosphere. This is subject to much variation in size, which at first seemed to lend support to the idea that a microsphere might be present in A. tenuis. To conclusively settle this doubtful point, however, it will necessitate the examination of a further series of specimens.
[Footnote] * Rep. Chall., Summary of Results, vol. i, 1895, p. 605.
[Footnote] † Archiv für Protistenkunde, vol. iii, 1903, p. 281.
The initial portion of the shell is subspheroidal, and seems to be merely an inflation of the tubular part of the shell. The dimensions of some of the initial chambers in their longest diameter are as follows: Specimens figured by Brady—360μ and 300μ. From the present series—100μ, 75μ, 50μ.