462 
BULLETIN 106, UNITED STATES NATIONAL MUSEUM. 
common form) are elongated, distinct, claviform; the frontal is convex, bordered 
by a line of round or triangular areolae; it is formed of a smooth olocvst, perfo- 
rated laterally, supporting a porous pleurocyst. The aper.tura is semilunar with a 
convex proximal border; the peristomice is semilunar; the peristome is little 
salient; there is no lyrula. The ovicell is rare, placed on the distal zooecia, some- 
what convex, completely surrounding the apertura, of the same nature as the 
frontal area. The median avicularium is peristomial and little elliptical, with 
pivot. On the line of the areolae there is another small elliptical avicularium, with 
pivot. 
Variations . — The zooecial variations are extraordinary ; they are occasioned 
by the pleurocystal calcification. The pleurocyst is developed above the olocyst 
(fig. 6) and may.be detached from it; the calcareous deposit, at first irregular (fig. 
5), is spread around the special pores (fig. 4) ; this is the most habitual aspect of 
the zooecia. The calcification increasing still more, the zooecia surround them- 
selves by a salient thread and with interareolar costules (fig. 8) and when the 
thickness is great there is no trace of the zooecial divisions (figs. 12, 13). It is 
easy to distinguish a porous pleurocyst from a tremocyst by the inspection of the 
zooecial interior; here (fig. 14) the interior is smooth and only perforated laterally 
by small pores corresponding to the areolae. 
The transversal section (fig. 10) shows the zooecial lamellae regularly super- 
posed; the very thick walls (fig. 9) are perforated by tubules allowing the zooecia 
to communicate among themselves. The oldest lamellae of the zoarium are filled 
up with calcite (fig. 11) and thus form a more rigid substratum for the later 
lamellae; we do not know whether this calcification is of chemical or physiological 
origin. 
The absence of the lyrula might permit this species to be separated in a special 
genus, but its mode of calcification is the same as all the other species of the genus 
S 'mittina. 
Its large zoarial dimensions render this species very easy to determine; it is 
a good sized and characteristic fossil. Nevertheless, small zoaria are not rare and 
there are even some bilamellar ones. 
Occurrence . — Lower Jacksonian (Mooclys marl) : Jackson, Mississippi (com- 
mon). 
Middle Jacksonian: Three and one-half miles north of Grovania, Georgia 
(rare). 
Jacksonian (Zeuglodon zone) : Bluff on south side Suck Creek, Clarke County, 
Mississippi (rare) ; Cocoa post office, Choctaw County, Alabama (very common). 
Upper Jacksonian (Ocala limestone) : Chipola River, east of Marianna, 
Florida; 14 miles above Bainbridge, Georgia (rare). 
Vicksburgian (Red Bluff) : Red Bluff, Wayne County, Alabama. 
. Geological distribution . — Rupelian (Stampian) of Germany (Reuss). 
Plesiotypes. — Cat. No. 64133, U.S.N.M. 
