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Show 384 SIR C. ELIOT ON NUDIBRANCHS [Mar. 1, the plumes do not resemble the ample and elaborate rosettes of Archidoris, Platydoris, kc. The arrangement and the approximate (but not the exact) number of branchiae will probably be found good specific characters in cases where a sufficient number of individuals has been examined; but I think that the number of plumes increases with age, at any rate in some species, and that hence the data furnished by a single specimen may be misleading. This is the case particularly in forms with a spiral arrangement, where it seems likely that the small plumes in the spire are developed later than the others. Thus in Chr. sykesi the number varies from 12 to 18 and in Chr. annulata from 9 to 16. In the latter species a spire is present in some cases and absent in others. The commonest number of branchiae seems to be about 10 (varying from 8 to 12 in individuals). A distinctly smaller number (3 to 7) is found in Chr. cardinalis, juvenca, elegantula, iheringi, krohnii, virginea, gloriosa, scurra, luxuriosa, albonotata, inconspieua, elegans, camama, riuiolphi, pantharella, and is indicated in the drawings of many other species. In Chr. punctilucens, lineolata, paupera, tryoni, bennetti, splendens, crossei, dalli, reticulata, godeffroyana, mollita, vicina, sykesi, and annulata the number of plumes is more than 12 and often exceeds 20. I have observed that in making a superficial examination of the living animal one is very apt to under-estimate the number of branchiae, inasmuch as the longer ones may project and the shorter ones be hidden. Hence the plates and descriptions of older authors cannot be considered as decisive on this point. 6. The labial armature consists sometimes of two plates and sometimes of a continuous ring. The elements are hardly ever straight, but are more or less bent or hooked. They are of very varying shape and thickness, and sometimes swollen just below the tip or terminal hook, so that they assume a mace-like appearance. The shape usually affords a good specific character, but in .some species the elements are bifid in one part of the armature and entire in another. 7. The radula offers many differences. Chr. scabriustnda has a central tooth and Chr. juvenca a central plate which is called " false " by Bergh, though it seems well developed. Rhachidian thickenings, more or less distinct, are found in Chr. cozrulea, iheringi, mbrchii, gonatophora, porcata, cameo, mariana, hilaris, sannio, lineolata, marenzelleri, cardinalis, reticulata, dalli, cavce, annulata, splendens. The radula is as a rule of moderate size, but is very narrow in Chr. elegantula, krohnii, gloriosa, and rather narrow in pustulans (75 x 28.0.28) and some other forms. In scabriuscula it is 108 x 30.1.30. It is unusually broad in thalcts-sopora (71 x 162.0.162), and above all in sycilla (81x290.0.290). The formulae for the radula given in m y descriptions are merely the shortest way of describing the teeth of a given specimen, and must not be understood as necessarily characteristic of the species. The proportion between length and breadth is generally roughly the same in different individuals, but the number of rows and of |