40 



the accessory portion. Within the division Anaska such a compound operculum 

 appears not only in the mentioned Chaperia species but also in Megapora ringens 

 and Foveolaria elliptica (PI. VII, fig. 7 b), but in these two species with the peculiar 

 modification, that the valvular portion and the accessory portion are here con- 

 nected by a low linear belt of thinner material in their whole breadth. 



Besides in the members mentioned of the division Anaska a compound operculum 

 appears in most members of the division Ascophora, and it may here be charac- 

 terized as a double operculum, as it really has a double function. While the 

 accessory portion of the operculum, which is placed proximally to the hinge-line 

 in the Anaska forms mentioned, is connected with the covering membrane of the 

 zooecium, it is continuous with the basal or inner wall of the compensation-sac in 

 the Ascophora, while the frontal wall of this sac is connected with either the distal 

 rim of the covering membrane (in the forms which possess a Cryptocyst), or 

 (where such is lacking) with the distal rim of the calcified frontal wall (Hippo- 

 thoidae and Catenariidae). This was already shown by Jullien\ who calls 

 attention to the fact that the hinge-line of the operculum does not coincide with 

 the proximal rim of the operculum, but lies more distally, whik the opening 

 which leads into the compensation-sac is placed immediately proximally to the 

 operculum. When the valvular portion of such a compound operculum emerges 

 from the zooecium, the accessory portion will on the other hand go down into 

 it and thus open the compensation-sac, to which it serves as operculum. In all 

 such cases where an operculum ends in a proximal convex rim or a rim furnished 

 with a projecting median poi;tion, as for instance in all the species referred to the 

 genera Lepralia and Schizoporella, it is evident that we have to deal with a compound 

 operculum, as a simple operculum must always end in a proximally straight or 



concave edge. On the other hand, the presence of such an edge does not exclude 



• 

 the possibility, that the operculum may be compound, and the only certain way 



to settle the question is to find the hinge-line of the operculum. An articulation 

 similar to that which takes place between the valvular portion of the operculum 

 and the accessory portion in Megapora ringens and Foveolaria elliptica occurs 

 within the division Ascophora in the genus Arthropoma, which includes the two 

 species A. Cecili and A. circinata, because the small proximal process of the oper- 

 culum is connected in a similar way with the valvular portion of the operculum. 

 We have already under the compensation-sac discussed the well-known fact, 

 that a series of forms have a median pore, placed at a greater or less distance 

 from the zooecial aperture, which leads into the cavity of the zooecium. It has 



' 45 a. 



