DITRYPA. 379 
tapers from the anterior to the posterior end. Occasionally several larger and smaller are 
in company, and they may touch each other or cross one another on the stone, and in a 
few the slender terminal region forms a loop in its progress. As a rule the tube is firmly 
attached to the under surface of the gneiss throughout its entire length. 
Marenzeller (1892) gives a figure of a jointed bristle and a hook. The outline of the 
bristle differs from that of the present form and from that of any known species, by having 
an indentation on the border of the serrated wing at the tip as if a kind of articulation occurred 
there. No bristle of this kind has been seen in Apomatus, though softened preparations 
occasionally simulate such a condition—especially under pressure. 
De St. Joseph (1894) found Folliculina ampulla in the tubes of Apomatus similis, Marion 
and Bobretzky—a species approaching the present very closely. Indeed, so far as can be 
observed, there is no valid reason for separating A. symilis from the common species. 
Wollebeek (1912) gives a figure which resembles in outline this species. His figure of 
the hook, however, differs, for instance, in the absence of the gulf behind the inferior spike, 
but his figure may be imperfect. 
Fauvel (1914, p. 361) considers Apomatus similis, Marion and Bobretzky, and A. globifer, 
Théel, as identical. The authors just mentioned place weight on the occurrence of a greater 
number of “ crystallins ” in the touches of pigment on the branchie, and on the presence of 
the hooks on the second instead of the third bristled seement, whilst they describe and figure 
differences in the bristles and hooks, the latter bemg devoid of denticles on the anterior edge, 
but such is either an abnormality or a misapprehension, and it is probable that the bristles 
slightly vary. 
On the whole, it is probable that the number of species of Apomatus may be considerably 
reduced in future, the modifications of the branchie and the differences in regard to bristles 
and hooks being due either to environment or misinterpretation. 
The Apomatus geniculata of Percy Moore! (1908) seems to be a closely allied and probably 
identical form. 
Miss Pixell’ (1920), in her account of the Polycheets of the Scottish National Antarctic 
Expedition, describes an Apomatus (A. browni) which has a small fin and a gap at the base 
of the terminal blade of the collar-bristles, thus differmg from the British species, and making 
an approach to the condition in the Spirorbids. De St. Joseph makes the uniform character 
of all the anterior bristles a special characteristic of the genus. 
Genus CLXXXIII.—Dirrypa, Berkeley, 1835. 
SYNONYMS. 
Dentalium, O. F. Miller, Montagu, Desh. 1810. Pyrgopoton, Montfiore. Conch. Syst., p. 394. 
1819. Ditrwpa, M. Edwards. Lamarck, ed. 2, v, p.637. 1825. Hntaliwm, Blainville. Man., pl., foss 
fig. 4, 1827. Brochus, Brown. Brit. Conch.,ii. 1829. Cresis, “Gadus,” Rang. Manual Mollusca. 
1829. Cresis Costa. Paleont. Neap. 1832-4. Ditrupa, Berkeley. Zool. Journ., v, p. 424. 
1 ¢Proc. Acad. Nat. Sc., Philad., p. 361. 
2 «Scott. Nat. Antarctic Exped.,’ vol. vii, Zool., p. 89, fig. 2a. 
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