A Revison of the Jurassic Bryozoa. 223 



Ilah. Congo (fioni between Stanley Pool and Lukolelc, 

 aiul tVoni TIj)i>to). Three females. 



Named in lionour of the Rev. V. (J. Harrison, who pio- 

 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 1. Llic 

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



I. The Specific Characters of the Cyclostomata. 



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 

 zoariuni, the shape of the orifices and of tlie zocecia, the 

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

 gonoecia, the arrangement of the avicularia and vibracula, 

 tile distribution of the spines and maculae, 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 cliaracters are available. We have to rely 

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

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

 (when ])rescnt), and the ornamentation of the wall. The 

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

 structure that tlieir 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 thai 

 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 J they have lumped together the forms of such different 

 geological horizons that, if their example be followed, the 

 study oi the group becomes valueless. 



To find a mean between tiiese extremes is not easy. Tlie 



