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

 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 within each lateral margin with a more or less 

 strongly developed muscular ridge. Immediately proximallj' to the aperture there 

 is in most species a large ouicularinni, the chamber of which often takes uji 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 oa>cial cover, have sometimes a membranous and 

 sometimes a calcified ectoocrcium. In the latter case there is in addition an oce- 

 cial cover formed in various ways. Multiporous rosette-i)lates, niore rarely multi- 

 porous pore-chambers. 



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

 P. marsiipiiun Mac Gill., P. margaritifera Quoy & Gaim., P. aciilimstris Smilt, 

 P. miniita Norm., P. comprcssn Sow., P. glaciata Waters, P. plana Hincks, P. Skciiei 

 Ellis & Sol., P. saccata Busk, P. injUita Waters (= P. laeuis Sniitt, pars), P. prin- 

 ceps Norman, P. tiibulifera Heller. 



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

 to the structure of the ocrcia, these in some species e. g. P. aciitiroslris, P. mar- 

 9 — - supiiim and P. margaritifera having a membranous ectooa'cium, whilst the ecto- 

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

 the calcified endoooeciuni gradually increases in thickness under the covering 

 membrane, an ooecial cover foimed 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. 



