W. D. Lang — The Reptant Eleicl Polyzoa. Q7 



BeptomuUelea tiiberosa, d'Orbigny (B.M. 36,746), pieces of some size 

 containing no avicularia might be broken, which, if found in the 

 first instance, would have been placed in the ' genus ' SemimuUelea. 

 Unless, therefore, a given zoarium is known to be complete, which 

 is not often the case, there is no means of knowing whether, 

 although no avicularia are seen, they did not occur in the lost 

 portions, because their distribution is rare and irregular, and their 

 presence or absence has not been discovered to be correlated with 

 any other structure. 



The distribution of closed zooecia is not generally sporadic like 

 that of avicularia, but when one occurs others are associated 

 with it, often in the same numbers as the normal zooecia, generally 

 more frequently, and often forming an area composed of closed 

 zooecia only. Like the avicularia their physiological significance 

 is a matter of conjecture,^ nor does the study of the rest of the 

 zooecia interpret their apparently irregular distribution. 



In his diagnosis of SemimuUelea irregularis, d'Orbigny, Gregory 

 remarks that the zooecia are " separated by ridges." This is very 

 clearly shown in most of the zoarium of the specimen B.M. D 4867, 

 but not so clearly, if at all, in other parts, and not at all in specimens 

 B.M. D 4818 and D 4843. Again, Gregory cites this character as 

 sometimes occurring in Beptelea puJchella, d'Orbigny. Forms 

 possessing it he calls var. plana. The character again occurs in 

 parts of the zoarium of the type^specimen of SemimuUelea Dixoni, 

 Lang (B.M. D 7845). It may be said, therefore, that ridges 

 between the zooecia occur as a character in various forms of reptant 

 Eleids, in scattered parts of the zoarium ; and that the distribution 

 of these areas is determined by unknown conditions. 



Parts of the zoarium containing abnormal zooecia may be found 

 outside the Eleids. The linear arrangement of apertures is specifically 

 distinctive of Proboscina radiolitorura (d'Orbigny). But parts of the 

 zoarium are liable to occur in which the arrangement of apertures 

 is irregular. A case of this (specimen D 975) is figured in the 

 British Museum Catalogue.^ 



In the Cretaceous forms of the genus Memhranipora, a Cheilostome, 

 the shape of the aperture is seen to vary in the same species. In 

 some instances this variation is clearly connected with Astogeny ; 

 the different shapes are growth stages. For example, a very 

 general sequence is as follows : — The first formed zooecia have 

 circular apertures, and approaching the distal parts of the zoarium 

 the shape of the aperture changes to oval, to ovate to very long 

 oval, nearly elliptical^; and katagenetic stages are shown. But 

 besides this zooecia are often found whose apertures are of a shape 

 differing from those in their immediate neighbourhood. 



Instances might be multiplied, but enough have been cited to 



1 J. "W. Gregory: British Museum Catalogue of Cretaceous Bryozoa, vol. i (1899), 

 p. 290. 



2 British Museum Catalogue of Cretaceous Polyzoa, vol. i (1899), p. 50, text-fig. 1. 

 ■■^ These terms are used \vith their botanical significance, as defined, for instance, by 



Asa Gray, " Structural Botany," 1879, p. 95. 



