424 A. W. WATEKS ON FOSSIL CHJLOSTOMATOUS 



first, towards the distal end, the zooecial oral aperture, which is 

 closed by the operculum ; and secondly, below it (proximally) an 

 opening leading to a space between an upper and lower membrane 

 on the front of the zooecium. To this I refer more fully when 

 dealing with C. alata and G. ampla, pp. 428, 429. 



Since my last contribution was published, a paper has appeared 

 from the pen of Dr. Jules Jullien *, which should be examined by 

 palaeontologists, as it deals with what are now understood as Mem- 

 branijporce, a group which it has long been felt must be divided up. 

 One division of this was very largely represented in the Chalk ; and 

 although we must not be misled into concluding that there were as 

 many species as the almost endless names of d'Orbigny would lead 

 us to suppose, yet a considerable number of species, in different 

 modes of growth, existed then. The most characteristic are those 

 with a thick calcareous expansion, which was covered, when living, 

 with a membrane in which was the opercular opening. The open- 

 ing of this calcareous expansion Dr. Jullien proposes to call the 

 " opesia," a term which will be useful, as aperture has been used with 

 various significations. In all my recent papers I have always 

 spoken of the orifice closed by the operculum as the oral aperture, 

 a term which I find it most convenient to retain ; and aperture I 

 have in this paper used alone in cases in which it is not quite clear 

 what we are dealing with; and, in examining fossils, we shall 

 sometimes find ourselves in this unfortunate position when we have 

 only small fragments to deal with, though probably, when there is 

 sufficient well-preserved material, we shall be able to decipher the 

 structure. This will be the case with such genera as Micropora and 

 Steganoporella. A very large proportion of the group with the 

 well-marked opesia have also large avicularian cells interspersed 

 among the zocecia, and Dr. Jullien proposes to call the group Ony- 

 chocellidse in consequence. 



There is no doubt that the shape of the avicularium in such cases 

 is a most useful character ; but I do not think that its presence or 

 absence can be made even a specific character, though it was made 

 a generic one by d'Orbigny. 



As supporting this, we may point out how important the shape 

 of the zooecial avicularium has been in specific determination of 

 Cellaria ; yet we often find large colonies without a single avicu- 

 larium ; and of the common Cellaria cellulosa I have had large quan- 

 tities through my hands in which, after calcining, I failed to detect 

 a single zooecial avicularium ; at other times I have only found a few 

 here and there ; on other occasions we may be more fortunate, and 

 find them abundant. So also with Membranipora angulosa, Ess., which 

 is living in the Mediterranean : sometimes the zooecial avicularia are 

 very abundant ; other pieces I have found with few, and some with 

 none at all. In the same way I have found chalk-fossils of this 

 group, some with and some without, but yet in mode of growth and 

 in every other respect similar. The genus Nellia was described as 



* " Kouvelle Division des Bryozaires Cheilcstoiniens," Bull. Soc. Zool. 1881. 



