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Show 250 MR. P. J. BELL ON THE ECHINOIDEA. [Mar. 4, acquaintance with his specimens; for he writes:-" The species of this section are most difficult to distinguish; they present several variations, which at first sight appear characters, . . . but these variations do not appear to be permanent in the specimens of the same habitat, but this fact requires verification with a larger series; the form of the fasciole is often different on the two sides of the same specimen " (p. 52). It is this statement of Dr. Gray's that gives a more accurate account of the real facts of the case, though he might have added, indeed, that the reentering angles vary greatly in depth. Of fifteen specimens which I have examined from the large series in the Museum, four have one reentering angle on either side in the anterior inter-ambulacra ; one has no angle on the left, and one on the right side ; one has one angle on the left and a shallow one on the right side ; four have one angle on the left and two on the right side; and five have two on both sides. With the series in m y hands I am therefore unable to come to any conclusion from Agassiz's second distinctive character. "With regard to the other point, the relative lengths of the anterior and posterior ambulacra, I have first to say that in no case that I know of are the anterior longer than the posterior ambulacra; and among such cases I reckon the representation given by Prof. Agassiz (pi. xxi. fig. 1) ; and, secondly, that of nine specimens selected, that in which the carinate character of the posterior odd interambula-crum was least well marked, had anterior ambulacra measuring 40 millims., and the posterior 43 millims., while in that in which the carination was most marked the anterior ambulacra measured 38 millims., and the posterior 40 millims. The following Table gives some details as to the just-mentioned nine specimens, which are arranged in an increasing order of carination, as judged by the eye, and are all apparently well-grown specimens, since all are more than 100 millims. in length:- Table I. Specimen. 1.... 2.. 3.'... 4.... 5.... 6.... 7.... 8.... 9.... Length of specimen. millim. 117 120 116 116 116 109 106 115 118 Length of ambulacra. Anterior. millim. 40, 40 35, 33 34, 34 32, 32 33, 33 39, 39 32, 32 34, 34 38, 38 Posterior. millim. 43, 43 36, 34 38, 38 36, 36 35, 36*5 41, 39 32, 32 36*5, 35 40, 40 Breadth of anal plastron. millim. 37 30 31 29 28-5 31 29 29 5 31 No. of inter-ambulacral angles. 1. r. 1 1 2 2 1 1 2 2 2 2 1 2 1 2 2 2 2 2 Locality. Naples. Naples. Naples. Samoa. Naples. In the next Table is given the proportions of five specimens from one locality, Naples; and it will fitly lead to the series of smaller forms. |