492 BULLETIN 106, UNITED STATES NATIONAL MUSEUM. 



nx. This species is not very variable in itself, but the frontal avicu- 

 larinin is so fragile that the fossils are generally deprived of it; it is replaced 

 by a large irregular cicatrix. 



A-ffin/t/cx. This species is rather close to Ponlla jnclcsonirfi. 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 cylindrical in its nonbordered, cylindrical 

 zooecia and in its smaller tremopores. 



It differs from Porella compactu, in which the zoarial fronds are identical, in 

 the thinness of the avicularium and in its more numerous and much smaller 

 tremopores. 



Occurrt nfc. Vicksburgian (Red Bluff clay) : Seven and one-half miles south- 

 west from Bladen Springs, Alabama (rare). 



Cotypes.Cat. No. 64301, U.S.N.M. 



PORELLA COMPACTA, new species. 



Plate 94. fljts. V2, 13. 



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 

 limbo, large, short, thickset, oblique, hiding a part of the aperture, costulated 

 longitudinally. 



(Ae=O.K'>mm. AperturaJAa=0.12mm. 



Measurements. Penstomice ' ... . ,i, 



U/tc 0.14mm. (interior) u=0.10mm. 



f 3=0.50-0.60 mm. 



Zooecia -L 



1 72=0.22-0.24 mm. 



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 Ponlla. 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. 



