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Show 1879.] MR. F. J. BELL ON THE ECHINOIDEA. 251 Table II. a CD J CD P-l 02 1... 2... 3... 4... 5... Length. millim. 132 120 116 115 71 Length of ambulacra. Anterior. millim. 42, 41 35, 33 34, 34 34, 34 21, 21*5 Posterior. millim. 42, 42 36, 34 38, 38 36-5, 35 26, 27 Breadth of anal plastron. millim. 37 30 31 29-5 22 No. of inter-ambulacral angles. 1. r. 2 2 2 2 1 1 2 2 1 1 Proportion of length of anterior to posterior ambulacra. 1000 to 1000. 1000 to 1028. 1000 to 1107. 1000 to 1073. 1000 to 1238. The differences in the length of the ambulacra observed in the last specimen seem to point to this character, so far as it is one, being variable with age. In the following Table four smaller specimens are compared, and seem to support this supposition :- Table III. The first two of these specimens were undoubtedly regarded by Dr. Gray as belonging to the species carinatus. a CD a •rH o CD 1... 2... 3... 4... Length. millim. 84 66 55 39-5 Length of ambulacra. Anterior. millim. 27, 27 20, 20 17, 16 12, 12 Posterior. millim. 31, 31 23, 23 21, 21 15, 15 Breadth of anal plastron. millim. 25 17 21 14 No. of inter-ambulacral angles. 1. r. 1 2 1 1 1 1 1 2 Proportion of length of anterior to posterior ambulacra. 1000 to 1144. 3000 to 1150. 1000 to 1234. 1000 to 1250. From these two tables we m a y perhaps draw the following conclusions :-(1) In specimens of Brissus longer than 100 millims., the proportion of the anterior to posterior ambulacra may be from To"#o" to "HHM- (2) I n specimens less than 100 millims. long the same parts m a y be to one another in the proportions from yy^y to yyf^y1. If, then, the characters above detailed are of no value or assistance in the discrimination of the species, we are led to ask what importance is to be attached to the possession of a keeled posterior interambula-crum ? its absence in young forms, its variability in older ones, and the littoral habitat of its possessors points rather to its being a character acquired by the individual itself than by the individual from its ancestors. In other words, the variations in carination seem to be such as are compatible with the general characters of the species ; it is a ready instance of the difficulty of discriminating between mere varia- 1 Verrill's measurements of his species B. obesus bear out this conclusion; his largest specimen measured 2 inches, the anterior ambulacra *65, and the posterior •80, giving thus a proportion of {°g,°, (and not, as by some curious slip, Verrill states of H C T M ] ) - |