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Show 102 SIR 0 . ELIOT ON NUDIBRANCHS [May 17, rhinophore; the first pair of cerata receive no branches at a ll; the second and third receive on the right hand a branch which bifurcates, and on the left a simple branch which, in the third, stops at the base of the cera and does not enter it. The remaining cerata receive no branches. I think these specimens are the B. digitata described by A. & H. and by Bergh. The best external character seems to be the tapering, finger-like shape of the cerata and of the process behind the rhinophores, to which no doubt the specific name is due. B o rn e l l a arbobescens Pease. [Bergh, " Neue Nacktschnecken, No. ii.," Journ. Mus. Godeffroy, Heft vi. 1874, p. 96; id. S. R. xvii. p. 886.] Several specimens from Mombasa Harbour. Note on living animals : " Yellowish white, with red reticulations on back and red tips to cerata." The alcoholic specimens are all much of the same size. All are whiter and more compressed than those of B. digitata, and the cerata are much smaller. The average dimensions are Length 20 millimetres, height 6, breadth 3 ; rhinophores and cerata about 2 mm. high. The tentacular processes at the side of the mouth consist of only about six digitations. The rhinophores are as in B. digitata, but the posterior process is not so long. In most specimens there are five pairs of cerata, of which the first three are bifid and the remaining two simple. Each, from the first to the fourth, bears two gills, the fifth none. The jaws and labial armature are as in B. digitata; the formula of the radula is about 40x9.1.9, rising sometimes to 12.1.12. The teeth are much as in Bergh's plates (Journ. Mus. Godef. I. c. plate iv. 12), but the central cusp of the median tooth is rather longer. The median tooth is more erect than in B. digitata, and the 8-10 denticles which it bears less distinct and very hard to see. The other characters are as in B. digitata. It is not easy to say whether this form is specifically distinct from B. digitata or, if so, whether it should be called B. arbo-rescens. It represents, however, at least a well-marked variety or stage of growth in which the tentacular processes, rhinophores, and cerata are less amply developed. It could hardly be identified with B. arborescens on the strength of the original description by Pease (Amer. Journ. of Conchol. vi. 1871), but in the revised description by Bergh (Mus. Godef. I. c.) the chief specific character seems to be " papillis anterioribus ut plurimum b ip a r t i t i s In these specimens they are invariably bifid. With regard to the hooks on the prseputium, I was unable to see the difference mentioned by Bergh, and found only simple or bifid hooks, not trifid, in both species. B o rn e l l a e xc e p ta B . (Plate IY. fig. 5 b.) [Bergh, Challenger Reports, Nudibranchiata, p. 36; id. Danish Ex p. to Siam, Opisthobranchiata, p. 202.] |
