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Show 1904.] FROM EAST AFRICA AND ZANZIBAR. 381 nearly all have a labial armature, and all * have teeth more or less differentiated. I confess that I a m afraid of attaching too much importance to the radula: a case like Aldisa, where a Dorid with otherwise ordinary characters has a unique dentition, certainly shows that the teeth may vary without any corresponding change in other characters; but in the genera here grouped together it will be found that the buccal characters are usually accompanied by some other feature which allies them to Chromodoris, such as simply pinnate branchiae or a long narrow shape. It m a y be said that the teeth of Dorids are never really uniform, and in the genera described in m y previous paper are often denticulate at the outer end of the row. This is true, but the outermost teeth are less well developed and more exposed than the others. They therefore have a natural tendency for purely mechanical reasons to become smaller and more irregular, and a particular form of this irregularity, due perhaps to some peculiarity of texture, is seen when they split up and become jagged or denticulate. But no such mechanical explanation will account for the innermost teeth being larger and more elaborately formed than the rest. Also this peculiarity is confined to certain genera, whereas the irregularity of the outermost teeth is general among the Crypto-branchiates and as noticeable in Chromodoris as elsewhere. Casella and Ceratosoma are clearly closely allied to Chromodoris, the former being perhaps not really a separate genus. Thorunna is practically Chromodoris without a labial armature. Aphelodoris has an elongate shape and narrow mantle-edge, but tripinnate branchiae and no labial armature. It seems, however, to be allied to Chromodoris by the presence of an accessory denticle on the innermost teeth. The remaining genera are of more or less oval shape, with a fairly wide mantle-margin, and papillae or tubercles on the back, peculiarities which are found in some species of Chromodoris. Sphcerodoris has simply pinnate branchiae and a radula which, though peculiar, is essentially of the Chromodoris-type. Orodoris, which Bergh associates with Sphcerodoris and Miamira, has the median part of the radula much as in Chromodoris. Holla t and Rostanga are allied to Chromodoris by their simply-pinnate branchiae as well as by their buccal parts : indeed, the former appears to m e almost an aberrant Ohromodorid akin to such forms as Chr. sykesi described below. In Rostanga the Mediterranean species perspicillata has denticulate inner teeth : in coccinea they are merely bifid. The buccal parts of Tyrinna, and Cadlina strongly resemble those of Chr. scabriuscula, which has also a somewhat oval form and tuberculate back. I somewhat doubtfully refer Audura to the same group, in virtue of its radula. This position is somewhat supported by its smooth skin and scanty bipinnate branchiae, but the structure of the foot suggests other affinities. * Except the very anomalous Miamira, which Bergh regards as allied to Sphcerodoris and Orodoris. f See note § on p. 380. |
