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because besides the varialion in the distal part of the aperture, we also have the 

 possibility for a so to speali endless variation in the extent and shape of^ the 

 sinus. That such a variation is not merely an abstract thought but really exists, 

 will be admitted by all who on the one hand have examined large quantities of 

 Bryozoa, and at the same time also have studied the considerable literature on 

 this subject. The majority of those species, which have a schizostome aperture are 

 referred by Hi neks and later writers to the two genera Schizoporella and Lepralia: 

 Though Hincks considers them to belong to two distinct families, it is in many 

 cases a matter of guesswork, whether to class a species to one of the genera or 

 to the other, because they can only be distinguished by a difference in the shape 

 of the aperture. Yet the diagnoses of the two genera seem to be quite different, 

 as a Schizoporella aperture is considered to have a sinus on the proximal edge, 

 but Lepralia a horseshoe-shaped aperture, contracted at the sides. Since, however, 

 such a proximal part of the aperture, so conti-acted, can in reality be regarded 

 as a sinus the difference between the two kinds of apertures is reduced to a 

 difference in the width of the sinus, and species with a narrow sinus have thus 

 been referred to Schizoporella, and those with a wide sinus to Lepralia. The 

 result of this consideration is then that the shape of the primary aperture, on 

 account of the practically endless variations to which it is subject in the Cheilo- 

 stome Bryozoa, cannot have any great systematic importance, and that it can at 

 the very most only be used as a more or less constant, auxiliary character in 

 the diagnosis of the genera. 



We arrive at the same result on considering the question from another point 

 of view. If we examine the aperture in a large number of forms belonging to a 

 series of families, we find that quite corresponding forms of aperture, holostome 

 as well as schizostome, reappear in all families which are rich in species, and 

 we may thus draw the conclusion, that these "different forms of aperture in each 

 of these families have arisen independently. To mention some of the most promi- 

 nent forms of aperture, we find for instance an aperture with a sinus in the 

 following families: Cribrilinidae (e. g. in Cribrilina clithridiata Waters), Thalamo- 

 porellidae, Myriozoidae, Escharellidae (in Schizoporella and Escharina), Smittinidae 

 (Smittina linearis, S. porifera etc.), Hippothoidae (Hippothoa, Trypostegd), Adeonidae 

 (several Adeonella species), Reteporidae (Retepora imperati, Ret. sinuosa, Rhyncho- 

 pora, i Schizoporella' scintillans, etc.), Catenariidae {Calpidium, Clauiporella, Hincks- 

 iella, etc.), Euthyridae (JJrceolipora nana) and Celleporidae. A semicircular aperture 

 with a simple operculum, which is furnished with a straight or slightly curved 

 proximal edge, is found further in the following families: Thalamoporellidae (Thai, 

 expansa, Thai. Jervoisi), Microporidae, Cellulariidae, Escharellidae (Microporella, 



