26 INTRODUCTION. 



P. desoudini series. 



p. c. I. r. 



P. desoudini (Haime) ... ... ... 1' 2 2 1-2 Jurassic. 



P. intermedia. Novak ... 1 1" 3 1-2 Cretaceous. 



.P. echinata (Miinst.) ... ... ... 211 1-3 Cainozoic. 



These formulae also enable us to estimate the general stages 

 of development in the Bryozoa at any period. The main line of 

 progress in the Cyclostomata has been from single encrusting 

 forms to those which are erect. Those which were able to raise 

 their crown of tentacles highest above the sea-floor were able to 

 obtain most food. Under the influence of the struggle for exis- 

 tence, there appears a constant tendency in the Cyclostomata to 

 secure some method by which the originally encrusting forms can 

 become erect or raised. Single series become multiple ; multiple 

 series spread into sheets ; and sheets give a sufficiently firm found- 

 ation for one part to rise as a frond or tuft. All through the 

 Cyclostomata we find the families beginning as encrusting linear 

 series or sheets, with low peristomes. Thus, the Tubuliporidae 

 begin with Stomatopora, the Idmoniidae with the adnate Idmonia, 

 the FascigeridaB with Defrancia, the Theonoidae with Actinopora. 

 In some cases the necessary increase of elevation is obtained 

 simply by the elongation of individual zooecia, or by the whole of 

 the distal portion being erect and free, as in Tubulipora. But 

 such isolated tubes are very readily broken off, and hence 

 the elevation is more permanently effected by the union of several 

 zooecia into tufts. But even in these cases there appears a 

 competition between the adjoining zooecia; and the influence of 

 the ever-constant effort towards an increase in the elevation of 

 the peristome may be seen. Hence the degree of increase in the 

 height of the mouth appears to serve as some measure of the 

 progress effected along any particular line of development. This 

 is seen in the case of the representative species of different 

 geological ages, as is shown in the following average formula of 

 the previously mentioned species of Stomatopora : 



p. c. I. r. 



Jurassic 1' 0" 1' 0' 



Cretaceous 1" 0'" 1' 0' 



Miocene 1" 1 1' 0' 



But the formulae for all the Jurassic and all the Recent species 

 of a genus show that the progress has, on the whole, been great, 

 though numerous cases of degeneration occur. 



