336 



tent and in different combinations. The most constant difference is seen in the 

 structure of the ooecia, but in this regard the genus Porella forms a connecting- 

 link between the two families, as a number of species of this genus have a mem- 

 branous ectoooecium and pore-chambers. 



Porella (Gray) Hincks. 



Spines are wanting or appear at most to the number of two. The primary 

 aperture has a straight, slightly convex or somewhat concave proximal rim, which 

 is often provided with a broad or low median tooth. Distinct hinge-teeth are as 

 a rule present. A distinct peristome. The operculum, which in the rarest cases 

 is somewhat strongly chitinized and sometimes not separated from the compensa- 

 tion-sac, is as a rule provided wdthin each lateral margin with a more or less 

 strongly developed muscular ridge. Immediately proximally to the aperture there 

 is in most species a large avicularium, the chamber of which often takes up al- 

 most the whole of the breadth of the zooecium, and its distally directed frontal 

 area is in time enclosed within the peristome. The ooecia, which have no pores 

 or at most a single one in the ooecial cover, have sometimes a membranous and 

 sometimes a calcified ectoooecium. In the latter case there is in addition an ooe- 

 cial cover formed in various ways. Multiporous rosette-plates, more rarely multi- 

 porous pore-chambers. 



Numerous species belong to this genus, amongst which are P- concinna Busk, 

 P. marsupium Mac Gill., P- margaritifera Quoy & Gaim., P acutirostris Smitt, 

 P. minuta Norm., P compressa Sow., P glaciata Waters, P. plana Hincks, P. Skenei 

 Ellis & Sol., P. saccata Busk, P. inflata Waters (= P. laevis Smitt, pars), P. prin- 

 ceps Norman, P. tubulifera Heller. 



The species may be divided into twcj groups (or perhaps genera) according 

 to the structure of the ooecia, these in some species e. g. P. acutirostris, P. mar- 

 supium and P margaritifera having a membranous ectoooecium, whilst the ecto- 

 ooecium most probably in most species is calcified. Whilst in the former group 

 the calcified endoooecium gradually increases in thickness under the covering 

 membrane, an ooecial cover formed in different ways may appear in the second 

 group. In P- struma and P- glaciata it is a single cryptocyst layer, which again 

 is covered by the covering membrane, whereas in P. saccata it is many-layered, 

 as thin calcareous layers, presumably gymnocyst layers, continually grow over 

 the ooecium, not only from the distal zooecium but also from the two neigh- 

 bouring zooecia, and we can see as a rule three, distinctly separated, thin cover- 

 ing plates on their surface. P. saccata ought perhaps to be referred to a distinct 

 genus. 



