POLYZOA. 613 



Palaeozoic Polyzoa appear to be for the most part referable to the 

 Cyclostomatous division of the Gymnolamata, and no unequivocal 

 examples of the Cheilostomata have been hitherto recognised in 

 any Palaeozoic formation. The form described by the present 

 author from the Ordovician rocks as Hippothoa ? inflata is refer- 

 able to Stomatopora (as this genus is usually understood), and in 

 that case is Cyclostomatous, as also is the Hippothoa devonica of 

 CEhlert. It is, however, possible that the Palaeozoic genus Pales- 

 chara, of Hall, is really Cheilostomatous. While the Palaeozoic 

 Polyzoa, therefore, must in the meanwhile be referred in a general 

 way to the Cvclostomata, it has to be borne in mind that many of 

 them exhibit very peculiar characters, and that certain forms are 

 apparently transitional between the Cvclostomata and the Cheilos- 

 tomata. It is, in fact, not improbable that fuller investigation of 

 the characters and structure of the Palaeozoic Polyzoa will render it 

 necessary to recast the classification of the Gymnolcemata ; and steps 

 in this direction have already been taken by Air Ulrich. It does 

 not seem possible, however, to accomplish this satisfactorily at 

 present, since we are not only still imperfectly acquainted with the 

 minute structure of many Palaeozoic Polyzoa, but there are various 

 Palaeozoic organisms, the precise place of which in the zoological 

 series has not yet been ascertained with certainty. This is notably 

 the case with the great Palaeozoic group of the Monticuliporoids, 

 which, for reasons previously given (see p. 352), have been here 

 placed provisionally beside the Coelenterates, while they are regard- 

 ed by Lindstrom, Ulrich, and other authorities as belonging to the 

 Polyzoa. It is also not impossible that we may have in the Palae- 

 ozoic rocks representatives of other groups of the existing Polyzoa 

 than the Cvclostomata and Cheilostomata. Thus, the singular 

 genus Ascodictyon, of the Silurian, Devonian, and Carboniferous 

 rocks, may possibly be, as suggested by Mr Vine, a representative 

 of the Ctenostomata, or may even be referable to the section of the 

 Entoproctous Polyzoa. 



The earlier Secondary deposits (Trias and Lias) have hitherto 

 yielded very few remains of Polyzoa ; but numerous forms of this 

 class are known in the later Jurassic and in the Cretaceous rocks, 

 the Upper Chalk, in particular, being extraordinarily rich in the 

 remains of these organisms. In the earlier part of the Secondary 

 period, by far the larger number of the known Polyzoa are still re- 

 ferable to the Cvclostomata, but in the later portion of the Cretaceous 

 period the Cheilosto?nata are likewise largely represented. In the 

 Tertiary rocks, again, not only are the remains of Polyzoa abundant, 

 but the majority of the forms represented are now referable to the 

 Cheilostomata, the Cyclostomatous types undergoing a decided re- 

 duction. All the great divisions of the Tertiary rocks have yielded 



