A Revison of the Jurassic Bryozoa. 223 



Hah. Congo (from between Stanley Pool and Lukolele, 

 and from Upoto). Three females. 



Named in honour of the Rev. F. G. Harrison, who pro- 

 cured this and many other interesting and rare insects during 

 his journeys on the Congo between the Stanley Pool and 

 Lukolele. 



XXVI. — A Revision of the Jurassic Bryozoa. — Part I. The 

 Genus Stomatopora. By J. W. Gregory, D.Sc., F.G.S. 



I. The Sjjecific Characters of the CycJoslomata. 



The diagnosis of species of Cyclostomatous Bryozoa has 

 always been regarded as a difficult and unsatisfactory task. 

 The Cheilostomata offer nine useful characters, some of which 

 appear to be very reliable. In this group the form of the 

 zoarium, the shape of the orifices and of the zooecia, the 

 structure of the front wall, the characters of the ooecia .or 

 gonoecia, the arrangement of the avicularia and vibracula, 

 the distribution of the spines and maculse, and the superficial 

 ornamentation give a combination of characters which enables 

 species to be defined with considerable precision. Unfor- 

 tunately in the typical Cyclostomata only the least trust- 

 worthy of these characters are available. We have to rely 

 only on the form of the zoarium, the length of the zooecia, 

 the size and position of tlie mouth, the shape of the ooecia 

 (when present), and the ornamentation of the wall. The 

 zooecia in the Cyclostomata are, however, so very simple in 

 structure that their characters are far less reliable than in 

 the more specialized subclass, the Cheilostomata. It seems 

 therefore at first sight almost impossible to diagnose species 

 while even the genera appear to vary to a hopeless extent. 



Two opposite methods of treatment have therefore been 

 adopted for the Cyclostomata. On the one hand, numerous 

 species have been founded on insignificant and individual 

 variations ; on the other, many authors have thought that 

 this subclass affords an illustration of the theory of the " per- 

 sistence of type," that was once applied, but has been 

 discontinued in the case of many other groups. They have 

 therefore abandoned the effort to separate species of different 

 ages ; they have lumped together the forms of such different 

 geological horizons that, if their example be followed, the 

 study of the group becomes valueless. 



To find a mean between these extremes is not easy. The 



