on the Polyzoa. 99 



terminal portion of the aperture. It is not therefore strictly 

 homologous with the orifice of the other Cheilostomata, which 

 is due to an arrest of the calcification of the front-wall. 



There is some variation in the position and character of the 

 two ribs which close in the orifice. In C. crassicosta, 

 Hincks *, two large stout spines originate one on each side at 

 the top of the cell, and bend round to the front, in the centre 

 of which they unite, inclosing a space of which the cell-wall 

 is the upper boundary. These spines are usually very broad 

 and represent the peristome of the sol id-wa lied Cheilosto- 

 mata. More commonly the two uppermost ribs of the costate 

 roof, which originate at some distance from the upper extre- 

 mity of the cell, constitute the boundary of the orifice in front 

 (the lower margin), whilst the cell-wall encloses it at the sides 

 and top. These two marginal ribs are thick and solid, and 

 at the central point of junction the extremities frequently 

 project and give a mucronate appearance to the front of the 

 orifice. This is often very marked, as in C. annulata } 

 Fabricius, and C. furcata, Hincks. Sometimes these mar- 

 ginal ribs do not meet exactly, and not unfrequently they 

 remain permanently disconnected; sometimes the extremities 

 seem to exceed the required length and are forced outward; 

 usually a small cleft may be detected, which marks the point 

 of junction. 



Dr. Jullien takes exception to my retention of those forms 

 with a quasi-mucron&te lower margin amongst the Gribri- 

 linidce, and considers that I am false to the principles on 

 which my classification is founded in not removing them to 

 the genus 31ucrone.Ua. That I have not done so he seems to 

 regard as an admission that the structure of the orifice is essen- 

 tially a character of inferior value as compared with the front- 

 wall of the zocecium, which he has adopted as the most 

 important for classificatory purposes. 



I do not propose at present to discuss the validity of the 

 mucronate margin in Mucronella as a generic distinction, but 

 merely to point out that it is by no means the structural 

 equivalent of the two ribs which close in the orifice of the 

 Cribrilinidce. The structures are totally dissimilar in their 

 morphological significance. The occasional and variable 

 prominence (for it is by no means constant where it occurs) in 

 certain species of Cribrilina, at the point of junction of the 

 ribs which compose the lower margin, and which is in fact 



* This is a very distinct form from the St. Lawrence, characterized 

 by having a small number of very massive ribs, which are separated by 

 wide intervals. In this species the protective covering is reduced to a 

 minimum. For description and figure see a paper on " The Polyzoa of the 

 St. Lawrence," ' Annals ' for March 1888, p. 216, pi. xiv. fig. 5. 



7* 



