16 



most of the families in this division; still I have not been able to find a cover- 

 ring membrane in members of the families Catenariidae, Hippothoidce, Eury- 

 stomellidae and Eathyridae, nor in the genera Inversiula and Anarthropora, and I 

 must therefore ascribe a Gymnocyst to all these forms. 



The two divisions Malacostega and Coilostega in reality evenly grade into one 

 another, and there is no doubt that the coilostegous forms have arisen from the 

 malacostegous by the extension of the calcification all over the frontal wall. In 

 some Thalamoporella species, for instance in Th. expansa (PI. VI b, fig. 5 a), the 

 operculum is surrounded by a completely calcified frame-work, while in most of the 

 species it is connected with a small membranous area posteriorly. We find a sim- 

 ilar relation between the species of the extinct genus Ehagasostoma and the 

 species of Onychocella, between CeUidaria and Membranicellaria and between Micro- 

 pora and such Membranipora species as Memb. argentea Mac Gill.S in which the 

 membranous area is only represented by an exceedingly small part proximally 

 to the aperture. The close connection between the Membraniporidae and the forms 

 now classed under Micwpora, Thalamoporella and Steganoporella, seems never to 

 have been doubted, and therefore older writers, such as Busk, simply refer such 

 species to the genus Membranipora. Hincks^ speaks about the relation between 

 the Membraniporidae and Microporidae in the following way: »In the most typical 

 forms, such as M. membranacea and M. Lacroixi, the entire area of the zooecium is 

 covered uniformly by a membrane, which lies a little below the level of the 

 .margin. In others this membrane is calcified to a greater or less extent, and a 

 solid lamina is thus formed, which protects a certain portion of the cell. But 

 even in such species, in which this process of calcification is carried furthest, 

 and almost the whole front is hardened into a solid wall, its position within 

 and below the marginal rim at once indicates the mode of growth, and reveals 

 the true Membraniporidan structure«. That Smitt has a similar view appears from 

 his placing the family Cellariidae, which has a completely calcified, depressed 

 frontal wall, in his suborder Flustrina. and from the following statement on the 

 family Microporidae^: )>Thus in the full development of the type, the primary 

 area, in the same manner as in Escharina, disappears, and as this was the most 

 pregnant character of the section Escharina, here, also, we perceive the close 

 proximity of that group, although yet the plain front side and the raised pri- 

 mary margins of the zocecia remind us of the Flustrine nature*. 



The answer to the question: whence the Ascophora have originated, is not 

 quite so evident, as we have here a compensation sac, of which organ no trace 



' 72, p. 179; ' 22, p. 128; ' 103, p 13. 



