xix FOSSIL FORMS GENERA OF MARINE POLYZOA $21 



existing families of Cyclostomata ; and one or two Cheilostomes 

 are recorded from the same period. Kecent papers by Walford 1 

 on Jurassic Polyzoa contain the description of genera which 

 are believed to be intermediate between the Cyclostomata and 

 Cheilostomata, particularly with regard to the characters of 

 their ovicells. Although it is not impossible the(re may be a 

 connection between the ovicells of these two groups, it has yet to 

 be proved that the two sets of structures are homologous. 



The Cretaceous period marks the commencement of a large 

 number of Cheilostome genera, although the Cyclostomes still 

 remain numerous. 



In the Tertiary formations the Cyclostomes gradually become 

 less numerous, and although in earlier geological periods they 

 far outnumbered the Cheilostomes, these relations are now 

 reversed. Certain Tertiary strata, and particularly the Coralline 

 Crag (Pliocene), are remarkable for the extremely large number 

 of Polyzoa they contain. It will be noticed that no mention, 

 has been made of the Entoprocta, the Ctenostomata, and the 

 Phylactolaemata. Their absence in the fossil condition 2 need not, 

 however, be a matter for surprise, as none of these forms are so 

 well suited for being fossilised as are the calcareous Cyclostomata 

 and Cheilostomata. There is consequently no adequate reason 

 for assuming that the absence of a palaeontological record implies 

 that these groups have been recently evolved. 



Determination of Genera of Marine Polyzoa. The species 

 to which a Polyzoon belongs can only be determined, in most cases, 

 with the assistance of the low powers of a microscope. There are 

 very great advantages in the use of a binocular instrument, by 

 means of which a microscopic preparation appears with its parts 

 standing up in proper relief. 



In the case of the calcareous forms, the external characters 

 may be more readily made out in a dry preparation than in any 

 other way. For this purpose, the colony should be washed with 

 fresh water, in order to remove the salts, which otherwise crystal- 

 lise out on drying and obscure the surface. Preparations of this 

 kind must be looked at with the aid of reflected light. Canada- 

 balsam or glycerine preparations are also valuable, whether 



1 Quart. J. Gcol. Soc. 1. 1894, pp. 72, 79. 



2 See, however, Vine, Ann. Nat. Hist. ser. 5. xiv. 1884, pp. 87, 88, and P. 

 Yorksh. Gcvl. Soc. xii. 1891, p. 74, for possible Palaeozoic Ctenostomes (Ascodictytn, 

 RJwpalonaria, and Vinella). 



