INTRODUCTION. 



29 



diagnosis of genera so difficult, but their classification remarkably 

 complete. Most of the well-known families can be arranged in 

 series, beginning with simple adnate forms and ending as erect 

 fronds or tufts. This is shown for three families by the following 

 table : 



The possibility of discontinuous variation, however, renders the 

 classification in some respects uncertain, until the limits of the 

 discontinuity are known. Thus, the Cretaceous and Cainozoic 

 Bryozoa, which are provided with cancelli, are here grouped into 

 a suborder, the Cancellata. But it is possible that the whole of 

 these genera are not descended from one ancestor, but that cancelli 

 may have arisen independently in different families of Tubulata. 

 Until this is settled, it seems wisest to leave the cancellate forms 

 in a separate group. 



1. The Four Orders of Gymnolcema. The Jurassic Bryozoa all 

 belong to that section of the subclass Ectoprocta known as the 

 Gymnolserna (p. 38). This is usually regarded as divisible into 

 three orders the Cheilostomata, Ctenostomata, and Cyclostomata. 

 The two first may be dismissed at once, as there are only two 

 Jurassic species known to belong to them, and these are both 

 members of the Cheilostomata. 



The last order of these three, the Cyclostomata, is defined by 

 Hincks as follows : " Zooecia tubular, with a plain, inoperculate 

 orifice ; marsupia and appendicular organs wanting." Three out 

 of the five statements in this diagnosis are negative ; and the only 

 positive characters are that the zooecia are tubular and the orifice 

 (i.e. aperture) is plain. But both characters are also met with 

 among the Cheilostomata, for Dr. Hincks commences his diagnosis 

 of the JEteidse with " Zooecia tubular," 1 and in the same family 

 the aperture is terminal and plain. 



1 Hincks. Brit. Mar. Polyz. p. 1. 



