654 BULLETIN 106, UNITED STATES NATIONAL MUSEUM. 



Generally the zooecia are either cylindrical or pyriform. In many of those forms which 

 have cylindrical zooecia throughout the greater part of the zoarium, the zooecia between 

 the first and third dichotomies tend to be slightly pyriform ; while in those forms with 

 pyriform zooecia, the zooecia between the first and third dichotomies are generally more 

 pyriform than the rest. 



Ribbing, when present, is usually faint at its first appearance, becoming stronger later 

 on, and in some cases becoming fainter again finally. The point at which the acme is reached 

 varies a great deal. 



Finally, Lang has demonstrated that the method of branching and the shape 

 cf the zooecia varies sensibly in time and according to the successive geologic 

 stages (fig. 215A). 



In the application of these observations Lang, in order .to characterize a 

 ppecies of Stomatopora, made a large table, 1 which is an excellent example of 

 bookkeeping. We are unfortunately not able to adopt this method in our work as 

 we do not possess a sufficient number of specimens of our various species. 



Before Lang's studies, Gregory, in 1896, had also devised a system of descrip- 

 tion, which, however, was given up in the succeeding volumes of the Catalogue of 

 the British Museum. In order to make the species collected in our American 

 Tertiary formations of stratigraphic value we have no other method than that 

 of good illustrations always on the same scale (X12 and X25). 



The branches of the same zoarium of Stomatopora or of Proboscviw. never 

 grow over each other (see pi. 105, fig. 1) ; a branch is arrested hi growth when it 

 encounters another. What is the mysterious force which permits the minute 

 branches of the same colony to be cognizant of each other in the eternal night of 

 the oceanic depths? Evidently this is a manifestation of a kind of cerebral activity 

 of which the nerve ganglion of the bryozoa is the organ. But how does the trans- 

 mission of the sympathetic vibrations occur and by what magic do they become 

 synchronized? Although microscopic the biologic mechanism of a zoarium is 

 sublime. 



STOMATOPORA OPPOSITA, new species. 



Plate 107, fig. 25. 



Description. The zoarium is not dichotomous ; the branches are opposed and 

 are emitted symmetrically at the distal extremity of a tube. The tubes are short, 

 finely punctate, elliptical. The peristome is orbicular, thick, little salient. 



Diameter of tube 0.36 mm. 



Length of tube 0.50-0.70 mm. 



Diameter of peristome 0.20 mm. 



Occurrence. Midwayan (Clayton limestone) : Mabelvale, near Little Eock, 

 Arkansas (very rare). 



ffolotype.C&t. No. 65247, U.S.N.M. 



1 Lang, Geological Magazine, vol. 2, 1005, p. 262 ; vol. 4, 1907, p. 23, etc. 



Measuremen ts. 



