84 



p. concinna, Smittina palmata, Discopora scabra and D. plicata. In the two last- 

 mentioned species there maj^ even in this regard be a difference between the 

 single zooecia in the colony. 



In connection with the aperture, we have still to mention as systematic 

 character the previously discussed vestibular arch, which seems to appear con- 

 stantly in the family Reteporidae and in most genera of the family Escharellidae. 



The peristome or the secundary aperture, which forms a more or less perfect 

 vestibulum to the true or primary aperture, appears only within the Ascophora 

 and is even within this division represented in a very irregular way, as it may 

 sometimes be absent in whole families (Catenariidae, Petraliidae, HoloporelUdae), 

 or genera (Microporella), whilst in others it appears to a very variable extent. A 

 peristome may sometimes be developed round the whole circuit of the primary 

 aperture {f Lepralia" canthariformis, the species of the genera Haswellia and Tuhu- 

 cellaria, Retepora pectinata Kirk, etc.), sometimes over only a larger or smaller 

 portion of this, being sometimes interrupted distally {^Phylactella«- labrosa, 'Ph.*^ 

 collaris, ^Schizoporella" armata Var.), or proximally (many species of Smittina and 

 Retepora). In some cases it is formed by continued growth of the edge of the 

 primary aperture (»Lep7-a/j«« Pallasiana Var, ^Lep.'^ canthariformis, Escharina sim- 

 plex), whilst in other cases it forms a wall outside this rim (} Phylactella<i labrosa, 

 »P/i.« collaris, "Schiz.i- armata Var). It may be low, circular (^Escharina sirnplex), 

 funnel-shaped ('Lep.«^ canthariformis), or tube-shaped (Retepora pectinata. Ret. 

 phoenicea, Escharella spinosissima, ^Phylactella- geometrica) and in a number of 

 cases provided with a pore on the frontal wall. It is occasionally furnished with 

 teeth-like projections, which in number and position are like those, which in 

 certain genera belong to the primary aperture. This applies for example to the 

 genus Exochella and certain species of the genus Escharoides. 



As the peristome is often very obvious it is easily understood why it has in 

 many cases been used as a systematic character at the cost of others more im- 

 portant, but less prominent, and in Hincks' great work the following genera are 

 besides the family Porinidae entirely or chiefly based on the structure of the 

 peristome, namely Porina, Lagenipora, Schizotheca, Porella, Escharoides, Smittia, 

 Phylactella, Mucronella, Palmicellaria and Rhynchopora. Of these genera I am only 

 able to retain Porella, in the limitation given by Hincks. 



In discussing the question of the systematic importance of the peristome, we 

 may first call attention to the fact that the same peristome forms recur within 

 a series of widely different families and genera. We thus find a collar-shaped 

 peristome, furnished with a frontal incision in many members of the family 



