416 



BULLETIN 106, UNITED STATES NATIONAL MUSEUM. 



below the frontal mucro in a locella. The operculum is slightly chitinizecl and not 

 distinctly marked off from the compensatrix. The peristome has three, sometimes 

 coalesced teeth, a median and two lateral. Normally there is an avicularium on 

 each side. Three dietellae. Spines. The frontal is surrounded by areolae. 



Genotype. Exochella longirostris Jullien, 1888. 



Range. Rocanean-Recent. 



Genus DIDYMOSELLA Canu and Bassler, 1917. 



1917. Didymosella CANU and BASSLER, Synopsis of American Early Tertiary Cheilostouie 

 Bryozoa, Bulletin 90, United States National Museum, p. 43. 



The frontal is a tremocyst. Below the aperture there are two large pores 

 which open into the zooecia, under the operculum. Spines. There is a large mar- 

 ginal avicularium, triangular, with pivot, arranged transversally. 

 Genotype. Didymosella (Porina) larvalis MacGillivray, 1868. 



Range. Vicksburgian-Recent. 

 We do not know the ovicell of this species 

 and we are ignorant of the function of the 

 two frontal pores. The latter have appeared 

 to us to have some relationship with those re- 

 sulting from the coalescence of the three labial 

 teeth of ExocJiella; and this simple relation, 

 combined with the presence of a large, margi- 

 nal avicularium, causes us to classify the 

 genus in the Peristomellae. In reality we 

 have not sufficient data. 



FIG. 122. Genus Didymosella 

 Bassler, 1017. 



A-C. Didymosella larvalis MacGillivray, 

 1868. A. Zooecia. (After MacGillivray, 

 1895.) B. Zooecia, X 12. (After Waters, 

 1882.) C. Avicularian mandible, X 85. 

 (After Waters, 1887.) 



DIDYMOSELLA CRASSA Canu and Bassler, 1917. 



Plate 88, figs. 1-7. 



1917. Didymosella crassa CANU and BASSLER, 

 Synopsis of American Early Tertiary 

 Cheilostome Bryozoa, Bulletin 96, United 

 States National Museum, p. 43, pi. 4, fig. 6. 



Description. The zoarium is unilamel- 

 lar and very thick; it creeps over algae. 



The zooecia are elongated, distinct, in the form of a bottle; the frontal is convex 

 and formed of a tremocyst with large, crowded, tubular pores. The aperture is 

 elliptical and transverse; the salient peristome bears the traces of very small spines; 

 two enormous pores are adjacent to the peristome. The avicularium is marginal, 

 triangular, very large, with pivot, and arranged transversally. The lower face is 

 smooth and presents some large, scattered concavities. 



Measurements. Aperture 



Aa=0.10mm. 

 , 

 Ua= 0.11-0.12 mm. 



Zooecia 



Zs=O.GO-0.70 mm. 

 fe=0.40mm. 



Variations. In longitudinal section we are better able to comprehend the 

 organization of this species. In particular the dorsal wall is a very thick olocyst, 



