86 



Yalve in Escharella and Escharoides we find a well-chitinized compound oper- 

 culum in Escharella polita and Escharoides sauroglossa. 



But the contrast between a simple and a compound operculum is of greater 

 systematic importance, and it seems as a rule to be an expression for a generic 

 difference. We find a simple operculum within a series of Ascophore families 

 {Eschar ellidae, Hippothoidae, Onchoporidae, Adeonidae^), and it is generally accom- 

 panied by a different mode of opening of the compensation-sac, which in these 

 forms with a simple operculum with a single exception (Chorizopora), opens out 

 through a median pore (see pag. 32). In spite of the above examples of varia- 

 tion, the structure of the operculum seems as a rule to show more constancy 

 within the genus than for instance the shape of the aperture and the peristome 

 and must be regarded as one of the best distinguishing characters. Referring 

 for the rest to the separate families, we may just mention as examples that 

 while a well-chitinized, compound, operculum is found in Schizoporella, Porella, 

 Cellepora and Adeona, a membranous operculum, which is not separated from 

 the compensation-sac, is the rule in the genera Escharella, Escharoides and 

 Discopora. The mode of attachment of the occlusor muscles seems also to be 

 rather constant within the genus, and we may cite as examples that there are 

 special muscular ridges or muscular processes in the genera Porella, Escharina 

 and Microporella, while Schizoporella, Cellepora, Arthropoma and Conescharellina 

 have muscular dots. 



The rosette-plates are interesting examples of a structural feature which, 

 though subjected in a number of cases to considerable variation within the genus, 

 and even within the species, proves in most cases to be constant, not only within 

 the genus, but also within the family. It is specially the rosette-plates of the 

 lateral walls, however, which show this constancy, as those which appear on the 

 distal wall in many cases show great variation, and we may cite the rosette- 

 plates on the distal wall of Membranipora membranacea as a very pronounced 

 example of such a variation (p. 24). Referring for the rest to the diagnoses of 

 the separate families and genera, we may here cite the following families and 

 genera, partly very rich in species, in which the rosette-plates of the lateral 

 walls especially show either perfect constancy in structure and appearance, or 

 only a small degree of variation, namely: Farciminariidae, Scrupocellariidae, Bi- 

 cellariidae, Cellulariidae, Steganoporellidae, Thalamoporellidae, Adeonidae, Reteporidae, 

 Catenariidae, Hippothoidae, Celleporidae, Holoporellidae, Onchoporidae, Conescharelli- 



' see pag. 38. 



