NORTH AMERICAN EARLY TERTIARY BRYOZOA. 



771 



character which would permit us to complete our knowledge of their biological 

 tendencies. This character is furnished us precisely in the study of the base of 

 the zoarium and in its mode of attachment to the substratum. This manifestation 

 of the ancestrular zooecia issued directly from the larvae is most important and 

 it will be necessary in the future to introduce it in descriptions. 



AI2 



D20 



FIG. 2o2. Genus Idmonea Lamonroux, 1821. 



A-D. Idmonea atlantica Forbes. A. Anterior face of the zoarium, X 12. B. Posterior face 

 basal lamella). X 12. C. Base of the zoarium, X 12. D. Ovicell and its oeciostome (o), X 20. 

 (After Harmer, 1915.) 



Species of Idmonea are often attached by an expanded base. The branches are 

 bushy with their dorsal turned toward the exterior. Very often the branches are 

 horizontal and parallel to the substratum, but this is a dangerous situation on 

 account of their fragility and liability to fracture. Certain species remedy this 

 by the addition of canals of reinforcement or firmatopores. Other species develop 

 on their dorsal, appendages of more or less length of a kind of calcareous radicell, 

 more numerous on the branches of the base, which strengthen the whole zoarial 

 system. Although the word "radicell" is not very appropriate, we have not thought 

 it necessary to coin a new name in order to designate these structures. 



The same species of Idmonfa often presents very different and unexpected 

 aspects which would lead one to believe in the existence of many species. The 

 micrometric measurements are in themselves insufficient. We have recourse to 

 two methods of identification and comparison the transverse section and the com- 

 parison of photographs enlarged upon the same scale. 



IDMONEA TACTA, new species. 

 Plate 107, figs. 19-24. 



Description. The zoarium is somewhat claviform, bifurcated, and much com- 

 pressed. The fascicles are salient, crowded, and formed of two to seven zooecia ; 



