The Megalospheric Form of Ammodiscus incertus

Abstract

THE interesting discovery of the megalospheric form of the above species in some abundance in the North Pacific Ocean, as described by Mr. J. A. Cushman in Bulletin No. 71, U.S. National Museum, 1910, pp. 73–5, and noticed. in NATURE of September 1, brings to mind the remarkable occurrence of the megalospheric form only (A. tennis, Brady) in some dredgings off Great Barrier Island, New Zealand, which I described in the Transactions of the New Zealand Institute in 1905 (1906). Curiously, the microspheric form was there entirely absent, although Dr. H. B. Brady had previously recorded it from a neighbouring Challenger station, No. 169. The latter author regarded A. tenuis as perhaps a local variety of the better known A. incertus. Rhumbler suggested that the form was possibly the megalospheric stage of the species, whilst the present writer, noting a large amount of variation in the initial chamber, suggested that a microsphere might be present in forms otherwise, to be regarded as A. tenuis, giving the diameter of the initial chamber in the New Zealand specimens as 100 μ to 50 μ. Mr. Cushman's published figure shows an approximate internal diameter of the proloculum as 250 μ, which, is nearer to Brady's published figures than to the examples from the Great Barrier Island. I am now convinced that the specimens from the latter locality had abnormally small megalospheres, giving the minima of measurements so far as known.