486 
BULLETIN 106, UNITED STATES NATIONAL MUSEUM. 
cave; the peristomice has the form of a trapezoid; the peristome is absent. The 
ovicell is small, entirely imbedded in the distal zooecium, not salient; its frontal 
is perforated with numerous small pores. The median avicularium is rather large, 
smooth; it makes a prominence in the peristomie in the form of a denticulated 
tube with its distal part. 
Measurements . — Peristomice j mm ‘ 
[lpe= 0.15 mm. 
Zooecia 
\Lz= 0.65-0.70 mm. 
I ?s=0.35 mm. 
Affinities . — Under the microscope it is rather easy to instinctively separate 
this species from Porella jachsonica , but upon analysis it is difficult to find the 
clearly separating characters. It appears to differ from it in the small denticles 
which often terminate the median avicularium and in the size and rarity of the 
tremopores. 
The tangential section (fig. 5) is that of a tremocvst resting on an olocvst; 
the ovicell is here confounded naturally with the peristomie. 
The tremopores are tubular and as long as the thickness of the zooecium. 
Occurrence . — Claibornian (Gosport sand) : One mile southwest of Rockville, 
Alabama. 
Lower Jacksonian (Mooclys marl): Jackson, Mississippi (common). 
Cotypes. — Cat. No. 64146, U.S.N.M. 
PORELLA JACKSONICA, new species. 
Plate 63, figs. 6-17. 
Description . — The zoarium is bilamellar, formed of two lamellae, back to back, 
difficultly separable; the fronds are flat or undulated- The zooecia are distinct, 
very elongated, fusiform; the frontal is convex and terminated distally by an 
enormous, oblique avicularian mucro making a projection at 45° and covering 
in part the apertura; the frontal is formed by a tremocyst with large tubular 
pores resting on a thin perforated olocyst. The apertura is transverse, semilunar, 
oblique, buried at the base of the peristomie. The peristome is not salient. The 
ovicell is entirely imbedded in the walls of the distal zooecia and is not salient; its 
frontal area is pierced with pores and very fragile. The median avicularium is 
smooth, enormous, very salient; it opens on the side of the apertura. There are 
some small, inconstant adventitious avicularia formed by the coalescence of two 
tremopores. 
{Jipe=0.10 mm. ^ . |'Zs= 0.54-0.80 mm. 
Measurements . — Peristomice 
[^6=0.10-0.12 mm. 
Zooecium 
Iz = 0.24-0.30 mm. 
Variations . — This species is of a disconcerting irregularity and it is with much 
trouble that we have been able to assemble the various forms which appear at first 
sight absolutely distinct. The irregularity arises chiefly from the alteration dur- 
ing fossilization, as is the habit in the species with great relief; the breaking of 
the median avicularium, the obliteration of the tremopores, and the rupture of the 
avicularian area are other causes which give to the specimens absolutely divergent 
aspects. Their illumination even under the microscope changes their aspect; the 
