136 BULLETIN 106, UNITED STATES NATIONAL MUSEUM. 



Variations. The micrometric variations in this species are very great: there 

 are long zooecia (figs. 7, 8), wide zooecia (figs. 9, 10), and dwarfed zooacia (fig. 7). 

 The furrow separating the zooecia is often filled and replaced by a thread-like 

 projection. Some cases of total regeneration have been noticed. The ovicell is 

 rare and fragile, its callosity is very finely granular like the mural rim. The 

 mural rim according to the rule is an olocyst in which the elements group them- 

 selves, radiating from the opesium. The gymnocyst is formed of scattered olocystal 

 elements (fig. 13). These tissues are very hard and it is very difficult to obtain 

 them in thin sections. Two very faint distal impressions exist on the dorsal olocyst 

 at the bottom of each zooecium. 



Occurrence. Middle Jacksonian : Wilmington, North Carolina (common) ; 

 Balclock, Barnwell County, South Carolina (rare). 



Cotypes. Cat. No. 62578, U.S.N.M. 



MEMBRANIPORIDRA SPISSIMURALIS, new species. 



Plate 27, figs. 1-19; plate 28, figs. 1-7. 



The zoarium is free, follicular. formed of two lamellae growing back to back 

 and easily separable. The zooecia are elongated, oval, distinct, separated by a 

 furrow or by a raised thread-like line. The mural rim is flat, oblique toward the 

 opesium, thick, gradually enlarging to form at the base a cryptocyst limited 

 laterally by two grooves or furrows. The opesium is elliptical, entire, often par- 

 tially cut off by the rectangular prominent denticles. The primoserial zooecia 

 have an opesium narrowed laterally. The ovicell is globular, salient, short, trans- 

 verse, and is deeply embedded in the distal zooecium. There are two pairs of 

 lateral septulae and a very large distal septula. 



. (Ao=0.45 mm. . |Z3=0.72 mm. 



Measurements. Opesia , /ooecia , . 



(fo=0.22 mm. {fs=().3o mm. 



Variations. This species is very common and. as usual in such a case, the 

 variations are innumerable. It is truly a protean form of the Membranipores. 

 The imagination can not conceive all the variations ; -we will note only the principal 

 ones. Certain of these variations affect not only an entire zoarium but often 

 all of the zoaria from the same locality. 



Our description is founded on the perfect but rare forms (figs. 1. 2, 3). The 

 cryptocyst may lack the lateral grooves; again it is concave (pi. 27, figs. 4, 5). and 

 then it is convex (pi. 27, figs. 6, 7). It may even be wanting entirely either natu- 

 rally (pi. 27, fig. 8) or by weathering (pi. 27, fig. 9). This latter variation affects 

 nearly all the specimens from Rich Hill, Georgia. 



The mural rim of young zooecia is thinner than usual (pi. 27. figs. 3. 12, 13). 

 Rarely the primoserial zooecia do not have an opesium laterally contracted (pi. 27, 

 figs. 1. 2). This contraction is a very important character, chiefly in the rolled and 

 worn specimens. 



We have observed monstrous zooecia from the Vicksburgian near Monroeville, 

 Alabama (pi. 27, fig. 14), and furthermore the extremely rare case of a zooecium 

 giving rise to five primoserial zooecia (pi. 27, fig. 15). 



