84 



P. coiuinixi, Smittina palincila, Discopora scabra and D. plicata. In llie two lasl- 

 nienlioned species there may even in lliis regard l)e a dilTerence between the 

 single zoa-cia in the colony. 



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

 character the i)reviously discussed vestibular arch, which seems lo appear con- 

 stantly in the family Rete})orid(ie and in most genera of the family EscIvircUidde. 



The peristome or the secundary ai)erture, which forms a more or less perfect 

 vestibulum lo 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 (Catenariiddc, Pdidliuldc. Holoporrllidae), 

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

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

 aperture (^Lcpralid" cfinthdrifonnis, the species of the genera Hnswellia and Tuhii- 

 cvlliirid. liclcpora prctiiuild KirU, etc.), sometimes over only a larger or smaller 

 portion of this, being somelimes interrupted distally (»P/ii//«c/e//a« hdirosa, »7-'/i.« 

 cnlldris, "Schizoporelki- drnudd Yar.), or [)roximally (many species of Smittiiid and 

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

 primary aperture ("Leprdlid P(dl(isidiui Xiw, »Lep.<^ cdulhnrijoimis, Esclvninii sim- 

 plex), whilst in other cases it forms a wall outside this rim ( Phi]ldclelld< lahrosd, 

 »Pli.' colhtris, "Scl}iz.« drmdki Var). It may be low. circular (Escluirinn siiuple.v). 

 funnel-shaped ('Lep.< canllidrij'orinis), or tube-shajjed (Retepord pectinala, Ret. 

 phoeniced, Eschnreltd spinosissititd, Phijlacleltd' (/eoiuctrica) and in a number of 

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

 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 Exochelld and certain species of the genus Eschdrnides. 



As the peristome is often very obvious il is easily undersh)od why il 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 Porinidde entirely or chielly based on the structure of the 

 peristome, namely Poriiui, Ldijempora, Schizotheai, Porelld. Eschdrnides, Snullid. 

 PluildctcUd, Mucronelld, Palinicelldiid and RliiinchoptH^. Of these genera I am onlv 

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



In discussing the cpiestion 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 dilferent families and genera. We thus find a collar-shaped 

 I)eristome. i'urnishcd wilh a frontal incision in many members of the familv 



