492 
BULLETIN 106, UNITED STATES NATIONAL MUSEUM. 
Variations. — This species is not very variable in itself, but the frontal avicu- 
larium is so fragile that the fossils are generally deprived of it; it is replaced 
by a large irregular cicatrix. 
Affinities. — This species is rather close to Porella jacksoniea. It differs from 
it in its much less oblique avicularium, its smaller tremopores, and its zoarial 
fronds much less wide and containing only from five to six longitudinal rows of 
zooecia at most. 
The young fronds differ from Porella cylindrica in its nonbordered, cylindrical 
zooecia and in its smaller tremopores. 
It differs from Porella comp acta,, in which the zoarial fronds are identical, in 
the thinness of the avicularium and in its more numerous and much smaller 
tremopores. 
O ccmrence. — Vicksburgian (Red Bluff clay) : Seven and one-half miles south- 
west from Bladen Springs, Alabama (rare). 
Cotypes. — Cat. No. 64301, IT.S.N.M. 
PORELLA COMPACTA, new species. 
Plate 94, figs. 12, 18. 
Description.-— The zoarium is free, bilamellar, formed of two lamellae, back 
to back, and easily separable; the fronds are very narrow and bifurcated. The 
zooecia are distinct, somewhat elongated, wide; the frontal is convex and formed 
of a tremocyst with tubules coalesced into very large tremopores, placed on an 
olocyst with numerous small pores. The apertura (interior) is elongated, semi- 
lunar, oblique; it has no salient peristome. The ovicell is very small, not salient, 
ornamented also with large tremopores. The avicularium forms an enormous 
umbo, large, short, thickset , oblique, hiding a part of the aperture, costulated 
longitudinally. 
-r. . , (A»e=0.16mm. Apertura \ha— 0.12 mm. 
Measurements. — Reristomice , Ai . , . N i 7 A1A m 
l^e=0.14mm. (interior) lto=0.10mm. 
„ . \Lz=0. 50-0.60 mm. 
ZooeC1 °ife=0.22-0.24mm. 
Variations. — In the interior the zooecial width (0.22-0.24 mm.) is evidently 
smaller than the exterior (0.30 mm.). 
There is no correspondance between the number of perforations of the olocyst 
(10 to 12) and the large exterior pores (5 and 6) ; that is, in fact, in their com- 
monage, the tubules become confluent and reunite among themselves. 
The mandible of the median avicularium is small, semicircular; it is placed 
at the extremity of the umbo and turned toward the apertura. 
Affinities. — 'This species differs from Porella erecta. in which the fronds are 
identical, in its umbo, which is large and short and not long and thin and its 
much larger tremopores. 
