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 iu 

 mind when the question of the range of distribution of this species 

 again comes under discussion ; the writer who theu 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 Asteroidea', 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 Strcngylocentrotus drobachiensis ; on the other 

 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, 



PSOLTJS 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 naturalists^ ih&t 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 Keren, 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. squainatus appears to be closer and the grains smaller ; and I have 



^ Jour. Linn. Soc. xiv. (1879) pp. 429-434. 

 ^ Cf. Duncan and Sladen, op. cit. 



