CRTBRILTNA ANNULATA. 195 



The mode in which the zooecium is developed in this 

 species is interesting as throwing light on all similar 

 structures, and on the affinities of the genus to which it 

 belongs. In its earliest stage it exhibits the simple 

 Membraniporidan form — the area occupying the entire 

 front, and being closed by a membranous covering. The 

 ribs, which compose the front wall in the adult, are given 

 off on each side as tubular processes from the edge of the 

 nascent cell, and, gradually lengthening, meet in the 

 centre and unite, the line of junction giving the subcari- 

 nate appearance to the zooecium. These tubular girders, 

 which are probably the equivalent of the marginal spines 

 on many of the Membranipora, are connected at intervals 

 by lateral outgrowths of calcareous matter ; and in this 

 way the porous structure of the furrows is produced. 

 This mode of formation may be well studied in the 

 thickened anterior margin of the orifice, which is com- 

 posed of two tubular pieces, the pointed extremities of 

 which in meeting often bend outwards, and give rise to a 

 central mucro. Sometimes they are not closely welded 

 together, but overlap one another, or remain partially 

 separate so as to give a bifid appearance to the mucro. 

 Occasionally they do not unite at all, but continue perma- 

 nently free and detached. 



The cells of C. annulata are not so intimately united as 

 those of most other species. "Aggregatae (nee tamen 

 connatse) " Fabricius says of them. They lie side by 

 side, but do not blend one with the other. Fabricius 

 describes them as occurring in a solitary condition in 

 Greenland. The walls are formed of very solid substance ; 

 and when fresh they are smooth and lustrous. 



The marginal spines are of two kinds : the lateral pair 

 are tall, and jointed to a short tubular process; the hinder 

 one is not jointed, but is a stout subconical outgrowth of 



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