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Show 1879.] MR. F. J. BELL ON THE ECHINOIDEA. 659 diameter. Taking note of this lapse, we will first consider those statements regarding the species in which the results to be gained from an examination of the British-Museum specimeus are not in complete accordance with the deductions of Prof. Agassiz :- (1) Comparative breadth of the poriferous zone.-Prof. Agassiz states (p. 501, s. v. H. variegata) that "the poriferous zone is also much narrower." As I found that m y own measurements reversed the relation, and led me to the conclusion that it was in T. esculentus that the zones were narrower, I have been at the trouble of reducing the figures in the ' Revision ' to a percentage value ; and I find them to be For " H. esculenta," 10, 8*3, 8*9, 7'7. For " H. variegata," 9*6, 8, 7. Pruned of its epithet " much" the statement of Agassiz is supported by his data. Tbe British-Museum specimens, which I have measured, do not exhibit so great a range of variation in the width of the poriferous zone, as may be seen from the appended list:- T. angulosus, 8, 8*2, 8*2, 8*6, 8 9 , 9, 9. T. esculentus, 7*8, 8, 8*4, 8*5. These observations indicate that the poriferous zones are rather narrower in T. esculentus than in T. angulosus; but they really run so close that it seems to m e that it is impossible to find in this character any constant or valuable point by which the two species may be distinguished. (2) Characters of the actinostome.-The relative size of the actinostome in Tripneustes angulosus as compared with that of T. esculentus is one of the few points of difference to which it is, as a rule, easy to point. In connexion with it there is another character, which it is perhaps safest to speak of as a tendency : in T. angulosus the actinal surface is, as a rule, perfectly flat, and the actinostome is flush with it, whereas in T. esculentus that same surface is ordinarily a little swollen, and the actinostome is placed in a shallow concavity. (3) The anal system.-It is interesting to compare the data afforded by the specimens of T. esculentus in the British Museum with those given by Prof. Agassiz. These latter are respectively 7'9, 8*1, 9*1, 9*2; those which are now given in the Table appended are 66, 7, 7*4, and 8 ; and they are to the point as leading us to insist a little more strongly on the comparatively smaller size of the anal system in T. esculentus than we should be justified in doing from a knowledge of Prof. Agassiz's measurements alone. (4) Difference in the size and number of the tubercles.-This appears to be a good character; but we must insist upon the fact that specimens of T. esculentus will be met with which have the median primary tubercles of the abactinal surface largely absorbed, while, on the other hand, there is in the Museum a young specimen which in the characters of its actinal and abactinal systems approximates to T. angulosus, but in which we find considerably well developed tubercles in the median spaces of the interambulacral areas. In |