288 
ELEIDiE. 
a spatulatc mandible,” says "W^aters of a Cenomanian specimen 
which he figures, “is distinctly indicated, and there can be little^- 
doubt that we have before us a vicarious avicularium. In some 
cases the end of the mandible has been unsymmetrical, similar 
abnormalities not being unfrequent in recent vicarious avicularia. 
In M. royana, Wat., there are also avicularia scattered over the 
surface, and here again, if we are to judge by analogy, we can 
scarcely doubt that in the beak there has been a chitinous 
mandible.” 
But avicularia arc still regarded as typical of the Cheilostomata. 
Waters^ speaks of “a character so distinctly Cheilostomatous as 
avicularia.” Harmer- states that “the avicularium and vibraculum 
. . . occur ... in certain Cheilostomata only.” Waters 
accordingly concludes ^ that “ in the Cretaceous Melicertitidae 
the characters are in the main Cheilostomatous united with somt^ 
that are Cyclostomatous.” In the Catalogue of the Jurassic 
Bryozoa (p. 157) I have referred to the MeUceritites figured by 
Ebmer as one of the Cheilostomata; and Waters seems disposed to 
assign the genus and its allies to that order rather than to the 
Cyclostomata. But this seems to be going too far ; for the 
structure of the zooecia is Cyclostomatous, as Waters admits. 
The resemblance to the Cheilostomata is due to the modification 
of some zooecia to act as avicularia. That this is a case of parallel 
development, and not of homogeny, is shown by the geological 
history of the group. 
The first stage in the development of these Cyclostomatous 
avicularia occurs in the genus Haplooecia, a Jurassic genus in 
which the zooecia, though tubular, present externally a certain 
resemblance to those of some Cheilostomata. Thus, in Saplocecia 
the zooecia, at their distal ends, are hexagonal, bounded by ridges, 
and have a small, subterminal aperture at the upper part. This 
arrangement is very similar to that of the genus Cellaria ; but the 
characters of the zooecia are essentially distinct. The branches of 
Haplooecia consist of a bundle of long tubular zooecia, whereas those 
of Cellaria are made of box-like zooecia, piled up like a brick tower. 
^ A. W. Waters. Chil. Char, in Melicertitida? : Ann. Mag. Nat. Hist. ser. 6, 
vol. viii. 1891, p. 49. 
2 S. F. Harnier. Polyzoa : Cambridge Nat. Hist. 1896, vol. ii. p. 524. 
^ A. W. Waters; op. cit. p. 53. 
