88 BICELLARIID^. 



it seems better to regard them as essentially identical (at 

 least provisionally) than to run the risk of introducing a 

 false species. Should further acquaintance with them 

 show them to be specifically distinct, the present form 

 would rank as B. uncinata. 



In some respects B. gracilis bears a considerable resem- 

 blance to B. plwnosa, but is of smaller size and of dis- 

 similar habit. It differs from it, too, in possessing three 

 spines instead of one, and in the character of the aperture. 

 The two free extremities of the margin above are more 

 prominent than in B. plumosa, the inner one especially 

 being carried forwards so as to be on a level with the 

 outer, and each of them being crowned Avith a spine. 

 The aperture is also much more decidedly turned inwards 

 towards the middle of the branch than in the latter species. 

 I have never met with the hooked appendage on B. 

 plumosa. 



In the number of the marginal spines B. gracilis agrees 

 with B. avicularia ; but they are very different in the two 

 species. In the former the uppermost of the two on the 

 outer side resembles the single spine of B. plumosa ; that 

 on the angular extremity of the aperture in front of it is a 

 strong denticle. The inner spine is not directed upwards 

 like that of B. avicularia, but projects at right angles to 

 the surface of the zoarium, and is placed exactly opposite 

 to the denticle ; the cells and avicularia of the two 

 species are also dissimilar. 



As I have mentioned, this form is furnished with a 

 very curious appendage, which must be regarded as a 

 modification of the radical tube. It originates on the 

 dorsal surface of the cell near its base, and stands out 

 from it at right angles. It consists of a rather stout 

 tubular process, which widens upwards from its point 

 of origin, and at its free extremity bifurcates, each 



