37 



manner as the above-mentioned chilinous region in Eiilhyris cUithiuUd bent strongly 

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

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

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

 which in a number of species of the genera Stetjanoporella (PI. V, ligs. 5 a, G 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 lis simplest form the operculum is a semicircular membranous valve, which 

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

 meets with the opercular arch. We might give a line running belween 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 Coilostef/a, as also in not a few members of the division Ascophora. 



In contrast to the oj)ercular 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 wilh 

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

 with the chitinous opercular arch. Besides in Clilidonia 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. e.vp(iiis(t (Pi. VI b, fig 5 b), Th. 

 iimmillaris, Th. Jervoisii (PL Via, fig. 4 c), etc., while in other Thalumoporellu 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, lig. 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 Oiujchocetla-spcvies (PI. XXII, rig.;U)) 

 and in some members of the family Scrupocellariidae, namely in Caberca Uoryi 

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

 the operculum is enclosed by a comi)lelely calcified rim, the proximal jiarl being 

 bounded sometimes by two projections from tlie calcified lateial parts of the 

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



• 107 a, p. .'•.1; " 17, p. 227. 



