85 



Retepondae, in a series of species of the genus Smittina, in Porella compressa and 

 Discopora Sarsi. A shorter or longer tube-shaped peristome with a pore on the 

 frontal wall is found in the genera Adeonella, Haswellia and Tubucellaria, in cer- 

 tain Retepora species (R. celliilosa. R. Couchi, R. complanata etc.), in Tessaradoma 

 boreahs, ^Porina'^ tubidosa, Smittina Lansborovi, var. personata), whilst a long 

 tube-shaped peristome without pores is found in certain Retepora forms (R. pedi- 

 nata, R. phoenicea), in certain Cellepora forms (Cellepora tiibulosa, C. bicornis), 

 Lekythopora hijstrix, ^Phylactellao. geometrica etc. Next, I would point out, as a 

 general result of my investigations on this point, that the peristome in many 

 cases is very inconstant within the genus (e. g. : in the genera Escharella, Smittina, 

 Discopora, Petralia, Cellepora, etc.), and Hi neks even mentions a series of cases 

 where the peristome in the same species may sometimes be present, sometimes 

 absent, which for example is the case in 'Lepralia^ Pallasiana, Smittina trispinosa 

 and Escharina Dutertrei. Whilst the above-mentioned small peristomial teeth are 

 found constantly in the small genus Exochella, their appearance is very incon- 

 stant in the genus Escharoides, and on the whole like Waters I cannot attach 

 very great importance to the peristome, which however does not exclude the 

 possibility that its apperance may be constant in a series of genera, e. g. in 

 Adeonella, Haswellia and Tubucellaria. 



The operculum. As all that has been said regarding the systematic impor- 

 tance of the form of the aperture, also holds good as a rule for the form of the 

 operculum, we may here merely take note of the other differences, which appear 

 in its structure. As already mentioned, we are able from the different relations 

 of the operculum to the frontal cover and the compensation-sac to distinguish 

 between an opercular valve, a simple and a compound operculum. These diffe- 

 rences have however only partially a systematic importance. We have a series 

 of examples of the fact that an opercular valve and a simple operculum may 

 appear within the same genus: e. g. in Steganoporella and Thalamoporella, and 

 especially in the last-mentioned we find a series of intermediate forms between 

 the two kinds of operculum. We have other examples in the previously men- 

 tioned species of Caberea and Scrupocellaria, in which the presence of a simple 

 operculum seems to be dependent on the strong development of the covering 

 spine. That an opercular valve and a compound operculum can appear in the 

 same genus, we have examples in the genera Adeona, Porella, Escharella, Escha- 

 roides, Smittina, Holoporella and Chaperia. Whilst for example we as a rule find 

 a compound operculum in Porella and Adeona, there is an opercular valve in 

 Porella Iceuis and Adeona violacea, and while there is generally an opercular 



