THE .VNNALS AND MAGAZINE OF NATURAL HISTORY, [SECOND SERIES.] No. 116. AUGUST 1857. VIII. — 'HeciocotyluB-formaiion in Argonauta and Tremoctopus explained by Observations on simila?' Formations in the Cepha-lopoda in ^eneraL By Professor Japetus Steenstrup*. [With two Plates.] This memoir is to be regarded essentially only as a somewhat detailed explanation of the figures on the two accompanying plates. The principal object of these figures is to induce naturalists to examine the animals themselves for the peculiarities to which they draw attention in a general way, rather than by their means to give an exhaustive picture of the details ; it would be better to reserve the latter for such figures as might be taken from living animals or freshly-caught individuals ; the present figures are therefore for the most part in outline, and only the par-ticular parts, of which a clearer representation was desirable, have been executed more in detail. The subject which they represent is an essential deviation from the symmetrical structure, otherwise so highly characteristic of the Cuttle-fishes, which has hitherto scarcely been observed, or, if observed, not sufficiently noticed, as we shall find that in all male individuals of that entire great group, one of the eight (four pairs) arms surrounding the head, on one side of the animal, is not only formed differently from that on the opposite side, but is even deve-loped in such a peculiar manner for a longer or shorter space of its * Kongelige Danske Videnskabernes Selskabs Skrifter, 5 Rakke, na-turv. og math. Afdeling, 4 Bind. 1866, Translated from the German of Professor Troschel, in Wiegraann's Archiv, 1856, p. 211, by W. S. Dallas, F.L.S. A7m, ^ Ma^. N, Hist, Ser. 2. Vol. xx. 6
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