416 
BULLETIN 106, UNITED STATES NATIONAL MUSEUM. 
below the frontal mucro in a locella. The operculum is slightly chitinized and not 
distinctly marked olf 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 Canlt and Basslek, Synopsis of American Early Tertiary Gheilostome 
Bryozoa, Bulletin 96, 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 transyersally. 
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 Exoehella; 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. 
DIDYMOSELLA CEASSA Canu and Bassler, 1917. 
Plate 88, figs. 1-7. 
1917. Didymosella crassa Canu and Bassler, 
Synopsis of American Early Tertiary 
Clieilostome 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. 
,, . [Aa=0.10mm. r , . \Lz=0. 60-0.70 mm. 
Measurements. — Aperture 7 A11 A1G ZiOoeciaA 
r 1^=0.11-0.12 mm. | \lz— 0.40 mm. 
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, 
Fig. 122. — Genus Didymosella Canu and 
Bassler, 1917. 
A-C. Didymosella larvalis MacGillivray, 
1868. A. Zooecia. (Alter MacGillivray, 
1882.) C. Avicularian 
(After Waters, 1887.) 
mandible, X 85. 
