THE MOliPHuLUGY OF THE GHElLOSToMATA. 285 



ovicells" found in certain species of F lustra and elsewhere^ 

 and in particular Dr. Levinseu's account of Alysidium 

 parasiticum, seem to be iu favour of the view that the 

 ovicell is a part of the fertile zocecium. 



Iu his original account of Flustra militaris/ Waters 

 suggests that this species is allied to E. episcopalis, calling 

 attention to the resemblance between the ovicells of the two 

 forms, but noting the existence of differences between the 

 opercula. The two species agree in the great length of their 

 zocBcia and in their rosette-plates ; but a striking* difference 

 is seen in the frontal surface, which is membranous in F. 

 militaris, except for a small proximal calcified region. 

 Parietal muscles appear to be inserted into this membrane, 

 as in an ordinary Flustra. The two strong suboral spines 

 of F. militaris may correspond with the two projections 

 which in B. episcopalis cut off' the median pore. If the 

 frontal surface has really the importance in classification 

 attached to it in the present paper, I see no way of admitting 

 the affinity of the two species (which undoubtedly resemble 

 one another), except by assuming either that F. militaris 

 is a species which has secondarily opened its compensation- 

 sac, or that E. episcopalis is a modified Flustrine form. 

 The material at my disposal is not in a condition which 

 allows me to make a further examination of these points. 



IV. The Morphology of the Compensation-sac and or the 



Operculum. 



I have come to the conclusion that the evolution of the 

 compensation-sac has not been identical in all Cheilostomes 

 which possess that structure, but that in some cases it has 

 been formed by the overarching towards the middle line of 

 a series of marginal spines in such a way as to cover the 

 primitive frontal membrane.^ The evidence for the occur- 

 rence of this process is as follows : 



(1) Many species of Membranipora exist in which the 



1 'Ann. Mal,^ Nal. Hisl.' (5), xx, p. 93. 



2 See p. 282. 



