THE BRYOZOA 



325 



which extend from the hitcral margins and converge, either to a central 

 line extending from the base of the zoecia or to the base of a raised 

 portion of the zoccial wall just below the aperture. An avicularium 

 is developed upon this raised portion, with mandible directed to one 

 side. CEcia ? 



This species seems to afford a transition between the Flustridai and 

 the Membraniporidte. It possesses flustrine characteristics in the 

 shape of the zoecium in the younger stages, and in the free frond-like 

 growth of part of the colony. Where it is attached, however, the 

 mode of adherence is membraniporidan. The thickened rim of the 

 zcecium grows fast to the substratum, and upon removal of the crust 

 the oblong shape of the under surface is left in outline. The type of 

 avicularia is that of Membratiipora^ as is also the secondary calcifica- 

 tion of a portion of the front wall. 



In a colony of moderate size, three stages of growth can easily be 

 distinguished. Upon the growing edge the zoecia are but faintly out- 

 lined, and the aperture occupies the whole of the front (fig. 9a, a/.). 

 This shape is retained through the next four or five rows, but the sec- 

 ond stage begins in the calcification of the lower half of the zoecia 

 and the strengthening of their lateral walls. Fig. ^b represents an 

 early stage of calcification. This begins sometimes as fine lines pro- 

 ceeding from the side and basal walls (/•), sometimes as thickened 

 growths resembling denticles (c^.). Soon the future aperture is out- 

 lined by the formation of a calcareous rim which does not quite unite 

 below it (fig. 9, ap.'). Meanwhile the calcareous thickenings along 

 the lateral (fig. 9, lat.) and basal (fig. 9, bas.^ margins of the prox- 

 imal end of the zoecia converge toward a smaller area (^arS)^ which is 

 left uncalcified just below the aperture. The secondary thickenings 

 gradually unite more or less, leaving quite wide spaces, or lacunae be- 

 tween them, which are covered only by the membranous material of 

 the original aperture. In the third stage, represented by PL XXI, fig. 

 10, a large sessile avicularium (az>.) is present upon the area below the 

 aperture. Calcification has continued, and the whole of the lower 

 portion of the zoecium has become involved. It is now covered by a 

 thin calcareous crust which slightly obscures the calcareous network 

 previously formed, and covers the muscular portion of the avicularium 

 with a delicate granular layer. The avicularium seems elevated upon 

 a kind of mound, and possesses a pointed mandible directed slightly 

 upward or in the direction of the rim of the aperture (fig. 10, man.'). 

 The mandible turns in some cases to the right, in others to the left. 

 CEcia are not known. 



