338 ESCHARID^. 



matter. At the extremity of the branches they are some- 

 what prominent, regularly oval and distinctly separated. 

 In this portion of the zoarium we find the primary orificCj 

 which is perfectly simple, without any trace of sinus and 

 on a level with the surface, instead of being, as it is in 

 later stages of growth, deeply immersed. The characteristic 

 aperture of the species, with the marginal sinus, is alto- 

 gether a secondary formation, which is developed as calci- 

 fication proceeds. The avicularium is connected with 

 the secondary orifice, and in the first instance is placed 

 on the surface, outside it and slightly overhanging it, 

 close to the sinus*. But ultimately it is inclosed by the 

 accretion of stony matter on the front of the cell, and lies 

 completely within the wall which has grown up around 

 the mouth. The various stages in this curious history 

 may be traced amongst the cells occupying the tips or 

 growing portions of the branches f. 



A striking feature of the species is the degree in which 

 the walls of the zooecia are thickened by the accretion of 

 calcareous matter, and the (apparent) rapidity with which 

 the process goes on. As a consequence the build of the 

 adult zoarium is solid and massive. 



In some cases numerous circular avicularia are distri- 

 buted irregularly over the cells, at times slightly elevated, 

 at times subimmersed. In the latter condition they are 

 no doubt being enveloped by the calcareous crust, and 

 would probably have disappeared altogether after a time. I 

 believe that in other instances the non-occurrence of these 

 organs, where they are usually present, may be due to a 

 similar cause. 



* It is so represented in Alder's figures of the cells accompanying Nor- 

 man's description of the species (Quart. Journ. Micr. Sc. /. c. pi. vi. figs. 11, 

 12). These figures are taken from cells in which the secondary orifice has 

 been recently developed. 



t They hare been accurately described by Smitt (/. c). 



