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Show 660 MR. F. J. BELL ON THE ECHINOIDEA. [June 17, addition to this it is to be borne in mind that there is now valuable evidence as to the fact that tubercles may, and do, undergo absorption *; so that we must not insist upon this character, where others point to the contrary species as being in our hands. (5) Ocular plates.-I had hoped that these structures would present some constancy of arrangement, which would be of assistance in the discrimination of the species, inasmuch as in the great majority of specimens of T. esculentus two only of the ocular plates reach to the anal system (or, in other words, are not shut out from it by the meeting of the edges of the genital plates ): thus of six examples all but one presented the arrangement just described, while the sixth had four ocular plates directly adjacent to the anal system. T. angulosus presented no such constancy ; for out of nine examples there were five that had two plates touching the anal system, while the others had three plates occupying a similar position. No conclusions can, therefore, be drawn from this character. Characteristic as is the arrangement of the pores in Tripneustes, it is only of assistance in the definition of the genus; when we come to any close examination we find, as indeed we might expect from what we know as to the mode of their development, that the arrangement of the pairs of pores with relation to one another varies considerably. I have noticed in large specimens of T. esculentus that the inner row of pores is quite regular, while the outer row is, as compared with it, irregular; in the smallest specimens the two flanking rows of pores exhibit very remarkable regularity, following one another in quite straight lines. The specimens exhibiting a pentagonal aspect come in very large quantities from the Red Sea; but there is in the Museum a specimen from the Philippines in which this form of test is just as well marked as in any Red-Sea specimen. There are some slight differences in the characters of the component parts of the dentary apparatus (lantern of Aristotle), which I will now proceed to indicate:- In T. angulosus the epiphysis is arched and its upper edge is bevelled ; the tooth is connected with the alveolus by delicate, but not very short, ascending and descending processes; the rotulae are short and broad; and the radii end in two short processes. In T. depressus the epiphyses are arched in very much the same manner as in T. angulosus ; the inferior ascending processes are of much the same character, but the superior processes are much shorter ; the radius is broadened out at its free end, but there is only a slight indentation at its extreme edge. In T. esculentus the epiphysis is less strongly arched, and its upper edge is not so sharply bevelled ; the tooth is connected with its alveolus by short pieces, which, above, are set nearly perpendicular to it; the inferior ones are only just seen through the triangular space, or, in other words, extend hardly at all upwards ; the rotulae are rather more delicate ; and the free end of the radius is distinctly 1 Vide Rev. of the Echini p. 265, Arbacia pundulata. |