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terminal partition-wall between two zooecia fits right opposite to a lateral wall in 

 an adjoining zocEcium. On the other hand, if two adjoining zocecia are placed 

 in a different relation to one another, the distribution of the rosette-plates and 

 the openings will also change. Even in those colonies where the quincunx 

 arrangement is most regular, we will sometimes be able to find places where 

 more or fewer zooecia are arranged in a less regular way. If now two adjoining 

 zooecia, for instance in Flustra foliacea, are so placed in relation to one another 

 that the one projects beyond the other by a quarter of its length, this quarter 

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

 has nothing but openings, which correspond with the same number of plates in 

 the adjoining zooecium. The law ought really to be expressed in this way, that 

 the part of the lateral wall of a zooecium, which extends beyond the distal wall 

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

 hind has openings. While most Cheilostomata 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 zooecia are principally arranged in 

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

 (Scbizoporelld) 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 Membranipora monostachys, and it is not unusual that a whole 

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

 Electra pilosa 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 this species, yet in a whole row of zooecia on the same 

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

 ever accompanied by another, as the lateral walls which bear the rosette-plates 

 are always much thicker and more strAigly 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 zooecia, 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 rosetle-plates 

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

 as well as on the frontal surface, and in the first case both in two-layered and 

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

 Smittina palmata (PI. XIX, fig. 5 b), Porella saccata Por. compressa, Thalamopo- 

 rella lioticha (PI. VI, fig. 7 i), Steganoporella magnilabris (PI. V, fig. 5 b), Dimorpho- 

 zoum nobile (PI. IV, fig. 1 c, 1 d), Microporella flabellaris (PI. XV, fig. 4 c) and Micr. 



