648 BULLETIN 106, UNITED STATES NATIONAL MUSEUM. 



mation characterizes the Cryptostomata, but it has also been noted in many different 

 families of the Cyclostomata and with varied forms of tubes, cylindrical (Cyr- 

 topora, Grammascosoecia, Crisina) or funnel-shaped (Gea). 



In biparietal gemmation the tubes in longitudinal section are, at their base, 

 adjacent to two other tubes. It is dorsal, axial, or intrazoarial. 



Dorsal gemmation occurs on the basal lamella. The tubes grow on the dorsal, 

 one from the other, early, before the formation of the polypide; the zone of 

 growth is visible and regular. The expanded tubes adopt chiefly this mode of gem- 

 mation (zoarial forms called Entalophora, Berenicea, etc.) in a large number of 

 families. 



Axial gemmation occurs by bifurcation in the vicinity of the zoarial axis. 

 The zone of growth is visible but irregular. It has no basal lamella, but there 

 is almost always an opposite plane formed of adventitious zooecia (Reteporidea, 

 Dcsineopora, Osculipora) . 



Intrazoarial' gemmation occurs at all heights and at all places in the interior 

 of the zoarium. The dorsal of the zoarium is not a basal lamella; it is formed 

 by the reunion of the tubes coming from the interior of the zoarium (Frondipora) . 

 There are all stages between axial gemmation and intrazoarial gemmation. In 

 tabular form gemmation is therefore as follows : 

 By juxtaposition. 

 Peripheral. 



(Triparietal. 



Oriented-- 



Biparietal 



Dorsal. 



Axial. 



Intrazoarial. 



ZOARIUM. 



Form. The form of growth in the Cyclostomata is quite variable and the 

 number of zoarial forms known is quite large. Generally the same species always 

 assumes the same zoarial form. Each form of growth has a particular name, 

 and in the old classification they constituted many distinct genera. The reader 

 will find these zoarial forms described in great detail in Gregory's Catalogue of the 

 Cretaceous Bryozoa. 



Basal system of -fixation. The basal system of fixation of the free zoaria is 

 little known. This is an absolutely new study to be undertaken when sufficient 

 material has been collected. The scarcity of such specimens has prevented us from 

 attempting this study. Certain accessory dorsal tubes undoubtedly form part 

 of the system of fixation and it will be useful to know their reciprocal rela- 

 tionships. 



Growth. The growth of the zoaria when there are no mesopores occurs: 



1. By zooecial superposition, if the tubes are cylindrical. 



2. By indefinite zooecial multiplication if the tubes are club-shaped. 



3. By superposition or agglomeration of subcolonies. 



The growth of the zoaria with mesopores is difficult to understand ; the section 

 of a small zoarium is identical with that of a large one. It is necessary therefore 

 to suppose that the animal constantly recommences its tube. This consideration 



