2.S 



lerminal parlition-wall helvveeii Iwo zod'cia (ils riglil ()pj)Osile to a latei'al wall in 

 an adjoining zoa'ciuni. On the other hand, if two adjoining zoa'cia are placed 

 in a diflerent relation to one another, the distribution of the rosetle-plates and 

 the oj)enings will also change. Even in those colonies where the (piincunx 

 arrangement is most regular, we will sometimes he ahle to find j)laces where 

 more or fewer zoa'cia are arranged in a less regular way. If now two adjoining 

 zoa'cia, for instance in Fhtxlra foliacea, are so placed in relation to one another 

 that the one i)rojects beyond the other by a quarter of its length, this ([uarter 

 will he provided with a rosette-plate, while the other part of the lateral wall 

 has nothing hut 0])enings, which correspond \\\[b the same number of plates in 

 tlie adjoining zocrcium. The law ought really to be expressed. in this way, that 

 the part of the lateral wall of a zoo'cium, which extends beyond the distal wall 

 of the adjoining zocecium, is furnished with rosette-plates, while the portion be- 

 hind has openings. While most Cheilostoumla are arranged in more or less 

 regular quincunx, there are on the other hand species in which this arrangement 

 can only be seen here and there, while the zoa'cia are principally arranged in 

 more or less regular transverse lines. This is for instance the case in Sinilliun 

 (Schizoporella) linearis, and most of the zooecia here will have either only rosette- 

 plates or only openings on the side-walls. There is also a strong inclination to 

 such an arrangement in Memhranipora nionostachys, and it is not unusual that a whole 

 row of connected lateral walls have either only rosette-plates or only openings. 

 Electra pilosd forms a peculiar exception from the common rule, and very likely this 

 is also the case with the other Electra species. Although as a rule we have the true 

 quincunx arrangement in tliis species, yet in a whole row of zoa-cia on the same 

 side we either find only rosette-plates or only openings. This difference is how- 

 ever accompanied l)y another, as the lateral walls which bear the rosette-plates 

 are always much thicker and more strongly calcified than those with openings, 

 and the last are very thin and after boiling in alkali often partly destroyed. We 

 can now and then find a whole row of zocecia, the lateral walls in which are 

 thick and furnished with rosette-plates, but then both the corresponding rows of 

 adjoining walls are thin and have openings. 



We have up to the present only discussed the appearance of the rosette-plates 

 on the vertical walls, but they may appear on the basal wall (posterior wall) 

 as well as on the frontal surface, and in tlie first case both in two-laj'ered and 

 in one-layered colonies. Thus, in two-layered colonies, I have found tluni in 

 SiuHtiim palinaia (PI. XIX, fig. 5 ))), Porella saccata Por. coinpressa, I'hdUujtopo- 

 rella lioiichu (PI. VI, fig. 7 i), Sle(janoporella magnilabris (PI. V, fig. 5 b), Diinorplio- 

 zoiim nobile (PI. IV, fig. 1 c, 1 d), Micrupurella llabellaris (PI. XV, fig. 4 c) and Micr. 



