OCR Text |
Show 1881.] PROF. F. J. BELL ON THE GENUS ASTERIAS. 509 Series y. Outer Skerries, Scotland. i. .. . 22=10*5, r=3-0, or ii 22=100, /- = 3*0, or iv 22- 9*0, r-30, or v B= 7*5, r=2*5, or Arranging these numbers in order, we 22=3*5 r. 22=3*3?-. 22=3-16/-. 22=3*0/*. 22 = 3*0/-. find for A. Breadth of ray. 3-5 3*5 4*5 3*7 3*0 ruBens 3*83, 3*6, 3-5, 3*25, 3*16, 3*16, 3*15 ; and for A. hispida 3*5, 3-4, 3*4, 3*3, 316, 3*0, 2*619, 2-437, 2-419, 2 0 . For A. ruBens, then, the proportion of R to r does not fall below 3*15: and the specimen in which that obtains is comparatively young, and has 72 only equal to 8*2 millim.; in A. hispida it may fall as low as 2, and that in a specimen of moderate size, or with a long radius of 12 millim. If we take only series a of A. hispida we find a marked shortness of ray ; but, unfortunately, that series is so far aberrant that the smallest specimens have B proportionally the longest. If, on the other hand, we take series y (and that is the series which Mr. Norman has presented to the British Museum), we find a set of proportions which can hardly be said to compare unfairly with series ft of A. ruBens. If now we take a series of older specimens of A. ruBens, we may be led to certain conclusions; the set taken, though not in any way specially selected, is very favourable:- i 22=65, r=13*5, or 22=4*81/-. ii 22=60, r=13'0, or 22=4*61 r. iii 22=56, /-=13*0, or 22 = 4*60/-. iv 22=46, r=120, or 22=3*83/-. W e are therefore, as it seems to me, led to the conclusion that for A. rubens there is a gradual increase, during growth, in the length of 22 as compared with r. In some cases, as with series y, sets of specimens of A. hispida may well fall into the series.; but, on the other hand, they may (as does series a) exhibit so great a variation as to lead to the suspicion that they must have been subjected to more or less abnormal conditions. And, finally, we cannot with safety appeal to the shorter rays of A. hispida as a definite and constant mark of specific differentiation. The next specific character given by Forbes is " rays rounded;" but the same term is applied to A. rubens ; then we have " spinous " for A. hispida, and " very spiny " for A. rubens. The last character, and one which is regarded as being a good mark of distinction in this genus, is the form of the " ambulacral avenues;" they are lanceolate in A. rubens, and "elongate-ovate" in A. hispida. As it is obvious that the form of the ambulacra will depend not a little on the width of the rays at their base, I proceed |