3^ 



manner as the above-mentioned chitinous region in Eiithyris clathraia bent strongly 

 basally and afterwards again frontally so that it forms a hood-shaped cavity with a 

 frontal concavity. This vestibular arch, which arises from the distal rim of the 

 primary aperture must not be confounded with the arch-shaped cryptocyst-ridge, 

 which in a number of species of the genera Steganoporella (PI. V, figs. 5 a, 6 a, 3 a) 

 and Thalamoporella is placed between the basal (horizontal), and the frontal 

 (obliquely ascending) portion of the distal wall. Waters^ who was the first to 

 speak about it calls it an oral shelf. It was later mentioned by Harmer^. 



The operculum. 



In its simplest form the operculum is a semicircular membranous valve, which 

 passes evenly over into the frontal membrane and is only chitinous where it 

 meets with the opercular arch. We might give a line running between the two 

 corners of the opercular valve as a border towards the frontal membrane, and 

 round this line, the hinge-line, the valve turns during the folding in and out of 

 the polypide. Such an opercular form can be found in most of the Malacostega, 

 in a number of Coilostega, as also in not a few members of the division Ascophora. 



In contrast to the opercular form just described, in other forms we meet with 

 an opercular valve which is separated in different ways from the frontal mem- 

 brane, and in the simplest cases by its proximal rim being furnished with 

 a chitinized thickening (basal sclerite, Harmer), which on each side is connected 

 with the chitinous opercular arch. Besides in Chlidonia Cordieri and most Stega- 

 noporella species (PI. V, fig. 3 c) such an operculum is found in a number of 

 species of the genus Thalamoporella, e. g. in Th. expansa (PI. VI b, fig. 5 b), Th. 

 mamillaris, Th. Jerooisii (PI. Via, fig. 4 c), etc., while in other Thalamoporella forms 

 the operculum is only partially separated from the frontal membrane by means 

 of a shorter or longer basal sclerite on each side (see PI. VI b, fig. 6 a). The 

 opercular valve can also be seen either entirely chitinized or calcified, and I 

 have already mentioned earlier the few recent species which possess a calcified 

 opercular valve. Within the group Malacostega I have found a completely chiti- 

 nized operculum both in some, not yet described Onyc/ioce//a-species (PI. XXII, fig.3b) 

 and in some members of the family Scrupocellariidae, namely in Caberea Borgi 

 and Cab. Darwini Busk and in a new Scrupocellaria species. In these three species 

 the operculum is enclosed by a completely calcified rim, the proximal part being 

 bounded sometimes by two projections from the calcified lateral parts of the 

 zocecium (C. Darwini), sometimes also by the here highly developed plate-like 



• 107 a, p. 51; * 17, p. 227. 



