43^ ItEPOET OF THE StATE GeOLOGIST. 



slightest change in the zooecium itself. In this species a number 

 of zooecia are scattered over the colony, which, whilst retaining 

 in great measure the usual form, are distinguishable by a re- 

 markable elongation of the oral valve. 



"This structure, which is normally semicircular in shape, is 

 here much produced and somewhat elevated above, and stands 

 out conspicuously on the surface of the zoarium. It is fully four 

 times the size of the ordinary operculum, and of course increases 

 Yery materially the length of the whole zooecium, which, in other 

 respects, departs very slightly from the normal condition. The 

 aperture is someiwhat reduced and the spines are aborted, but in 

 general, these abnormal cells very closely resemble the other 

 members of the colony. The polypide in such cases is probably 

 suppressed. We have here, it would seem, one of the earliest 

 and simplest departures from the normal type of the zooecium in 

 the direction of the avicularium. In Flt:stea the change is 

 much more marked, as the cell is merely rudimentary and the 

 movable operculum constitutes the essential feature. These 

 slightly specialized forms, which fill the place of the zooecium in 

 the colony, may be distinguished as ^primary avicularia . 



"They occur under various modifications. In SchizotheGa fissa 

 the avicularium has an area of the same size and form as that of 

 the zooecia, the beak and mandible occupying much the same po- 

 sition as the oral opening. ISTo where is its morphological signifi- 

 cance more apparent. A striking case of the same kind is pre- 

 sented by Gellaria tenioirostris. In Schizo^orella vemista, on the 

 other hand, the area (which replaces a cell) is reduced to a very 

 diminutive size and has a minute rounded mandible. The next 

 marked stage in the developmental series is characterized by the 

 contraction of the area combined with the assumption of a more 

 or less peduncular character by the hollow portion of the struc- 

 ture. The external resemblance to the ordinary z oecia has dis- 

 appeared ; the cell is commonly represented by a subcorneal ele- 

 vation, on the summit of which are placed the beak and man- 

 dible. At the same time the avicularium is now, for the most 

 part, a secondary growth and is developed not on the original 

 plane of the colony, but on the zooecia themselves. There has 

 been a large reduction in the size of the chamber, no longer re- 



