OCR Text |
Show 1882.] PROF. F. J. BELL ON THE GENUS PSOLUS. 645 be noted (1) that there is just as much imbrication in a large specimen from Massachusetts Bay, collected by the United-States Fishery Commission, and presented by the Smithsonian Institution ; (2) that taking the three Greenland specimens already referred to, we find a most obvious relation between the size of the example and the extent of the imbrication of the scales, the latter decreasing as the former increases ; (3) that the same phenomenon is to be observed in the series of Japanese specimens, a study of which leads one to the conclusion that the increase in the covering-capacity of the bivial armature is, at any rate, partly due to a diminution in the extent of the overlap of the different plates,. At the same time it is to be remembered that the Japanese specimens examined are all smaller than any one of those from the Atlantic which I have had the opportunity of comparing with them. And this must be borne in mind when the question of the range of distribution of this species again comes under discussion ; the writer who then treats of the matter will not, I trust, fail to carefully study the philosophical remarks on this subject which are to be found in Mr. W . Percy Sladen's account of Captain St. John's Japanese Echinoidea and Asteroidea1, where the importance of distinguishing the characters of forms with a wide distribution is most wisely insisted on. In addition to the indications of a wider distribution than was suspected for this species, the preceding discussion brings also into prominence the fact that younger are more strongly imbricated than older specimens, but that, so far as we can judge from a single example, the American race retains more than the European this overlapping of the plates. Just as the appearance of Psolus fabricii or a most closely allied form in the Japanese seas is a matter which need excite no wonder, so the second locality whence, as I fancy, the species is now for the first time recorded, only brings the species into the category of such circumpolar forms as Strongylocentrotus drobachiensis; on the hand, now that we know that the species is to be found at Kamt-rhatka, we are able to accept, with, as it were, a kind of personal experience, the fusion of P. sitchaensis, Brdt., with P. fabricii. PSOLUS SQUAMATUS. Two magnificent specimens of this species, the longest 130 mm. long, from the Gulf of St. Lawrence, were presented in 1880 by Principal Dawson ; and their examination brings to mind the view of some naturalists2 that P. fabricii is nothing more than a variety of it. At the first sight of the British-Museum specimens, such a view would be warmly rejected; but now that we have learnt the kind of changes that occur in imbrication during growth, there would be no reason to imagine, even if we had not the figure of Koren, that P. squamatus in the young condition has the plates less imbricated than P. fabricii. On the other hand, the granulation of the scales in P. squamatus appears to be closer and the grains smaller; and I have 1 Jour. Linn. Soc. xiv. (1879) pp. 429-434. 2 Of. Duncan and Sladen, op. cit. |