IMEODUCTIOX. 
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3. Cheilostomata, of which two species are known from the 
Jurassic, are represented in the Cretaceous hy a large fauna, 
which begins with the simple Athyriata, and includes in higher 
Cretaceous horizons members of the chief divisions of the Order. 
The Cheilostomata are the predominant Bryozoa in existing seas, 
and first became important in the Upper Cretaceous. They began 
in the Jurassic, hut in that system they are extremely scarce. 
There are, in fact, still only two known Jurassic species, and until 
others are discovered to connect them with the Cretaceous fauna it 
is impossible to avoid some suspicion as to the horizon from which 
they came. The Jurassic Bryozoa are still so little known that 
there appears no adequate reason to disbelieve the Museum labels 
as to the source of the Museum specimens of the Jurassic 
Cheilostomata. They belong to the Tesson Collection, are duly 
labelled, and resemble llanville material. Moreover, one of the 
species was recorded by Lamouroux from the Bathonian of Eanville. 
Nevertheless, those two species are so isolated that they afford 
a striking instance of the imperfection of the palaeontological 
record ; and, until other Jurassic Cheilostomata have been found, 
it is impossible to avoid some doubt as to their horizon. 
Descriptive Nomenclature. 
Many of the general questions connected with the fossil 
Bryozoa, their typical structure, and the chief terms used in 
their description were dealt with in the Introduction to the 
Catalogue of the Jurassic Bryozoa (1896), but further explanations 
are necessary in respect to some of the terras. 
OvicELLs. — The term ‘ ovicell ’ seems to me best used in a 
general sense for the various structures developed in Bryozoa for 
the protection of the ova. It may thus be used to include the 
ooecium of the Cheilostomata, the gonoecium of many Cyclostomata, 
and that type of ovicell for which the name gonocyst was 
proposed in the Jurassic Catalogue. These three structures seem 
distinct in character and origin, and are therefore best described 
by special names. 
The term ‘ gonocyst’ has been criticiaed hy Dr. S. F. Harmer^ ; 
but the distinction between the three types has been strongly 
^ S. F. Harmer. “Notes on Cyclostomatous Polyzoa”: Proc. Camb. Phil. 
Soc. vol. ix. pt. iv. 1896, pp. 211-12. 
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