30 



might be leucocytes, which can often be loimd fixed to the inner surfai-e 

 of many rosette-plates and which can easily l)e taken for pore-areas. Kiithijris 

 ohtecta (P\. XV, fig. 2 a) and UrceoUpora nana Mac Gill. (PI. XV, fig. 1 a) are also 

 furnished with rosette-plates, and the marginal |)ores, which ap[)ear in a single 

 or ilouble row in numerous Cheilostomata, seem always to be rosette-plates, which 

 are usually furnished with a single pore-area. We can for instance mention 

 Escharella immersa, Escharoides coccinea, Porella strnma and Smittina palmata. 

 These rosette-plates are always without a pore-ring and are (|uite membra- 

 nous, for which reason they quite disapi)ear when boiled in alkali. Some- 

 times in many species — for instance in the species of Celleporn and Holo- 

 porelhi — they may come to lie at the bottom of shorter or longer canals, 

 partly by the calcareous wall's growing in thickness, partly i)ecause the calci- 

 fication takes place in such a way that the canals leading to these rosette-plates 

 pierce the calcareous wall under very pointed angles. Such long canals can for 

 instance be found in Tessaradoma borealis and Escharella spinosissinm^, and espec- 

 ially in the last species they are remarkable for their considerable length, so 

 that even some of them may reach from the marginal portion almost right in to 

 the middle line of the frontal wall. In other species these marginal pores are 

 enclosed or overbuilt by small calcareous spaces which are furnished with a 

 larger or smaller opening and which we may compare with the above-mentioned 

 pore-chambers. We may mention Sniittina reticulata, Sm. palmata (PI. XIX, fig. 5 a), 

 Escharella variolosa, Discopora verrucosa and Disc, pavonella as examj)les of species 

 which possess such well-developed marginal cavities or areola'. The three or four- 

 sided areolae are outwardly limited by a projecting line, which is simply a contin- 

 uation of the lateral wall of the zocecium, and are separated from one another 

 by a number of transverse buttresses, which grow in length with age and in 

 older zoa^cia even touch one another in the middle of the zoa>ciuni. Two ad- 

 joining lateral ridges will, as is the case with the lateral walls from which they 

 spring, after boiling in alkali solution separate from one another, and if we look 

 at such a separated row of zoo^cia from the side we see these lateral spaces 

 through the wall as light canals, which on superficial observation would seem 

 to belong to the lateral walls. Still we must remember that the rosette-plates 

 which lie at the bottom of these spaces are really placed on the frontal wall. 

 It is not in all cases however that the superficial pore-chambers are externally 

 hounded by such a projecting ridge; this is not the case e. g. in Escharoides 

 Jacksoni, in which species these spaces are short, sac-like with an aperture facing 



' 34. PI. III. fig. 3. 



