308 RIDNKY F. HARMER. 



openings a sharply marked curved line (.»') indicates the edge 

 of a calcai'eous phite simihir to tliat described below in C. 

 plagiostoma. The markings between the fenestree and the 

 operculum are somewhat variable. In a specimen which 

 partly dried up during its preparation, fine slits containing 

 air were noticed, radiating from the fenestras to the orifice. 



The compensation-sac of this species is described by 

 JuUien (1888, 3). It is well developed, but its complete out- 

 line is not easily seen in the mature zooccium in consequeuce 

 of the fact that its breadth is greater than that of the exposed 

 scutiform frontal wall. 



Catenicella plagiostoma, Busk, var, setigera, 



MacGillivray^ (fig. 54). 



This species is remarkable for the obliquity of its orifice, 

 and for the enormous development of its avicularia, which 

 may have very different forms, even on opposite sides of the 

 same zooecium (fig. 54). The infra-avicularian compartment 

 {inf. civic.) is almost entirely closed by membrane, the single 

 fenestra so formed extending mainly over the side and basal 

 surface of the zooccium. The supra.-avicularian compartment 

 {sup. avic.) similarly has membranous walls, a very large 

 fenestra being common to it and the avicularian zooecium. 



The structui'e of the frontal wall is more easily made out 

 than in C. alata. The outer calcareous layer is reduced to a 

 system of conspicuous bars which unite with a calcareous 

 framework surrounding the operculum. The internal 

 calcareous layer is an obliquely oval plate" {pi.), which projects 

 towards the proximal end of the zotjccium. This forms part 

 of the roof of the compensation-sac, from which muscles {'p. in.) 

 can be seen radiating out to the adjacent parts of the wall of 

 the zoa3cium. The distal groups of parietal muscles {jhvi/} 

 appear to act as divaricators. 



In a back view of a zooecium it is seen that the avicularian 

 zooecium has a very hii-ge oblique fenestra proximal to the 



' ' I'rodr. Zool. Vict.,-" Dec. iii, 1879, p. 17. 



• Tills structure is described by Waters (1883, p. 429). 



