286 BULLETIN 106, UNITED STATES NATIONAL MUSEUM. 



MEMBRANIPORELLA CRASSICOLLIS, new species. 

 Plate 4, figs. 24, 25. 



Description. The zoarium is bilamellar and free. The zooecia are little dis- 

 tinct, being confluent by their mural rims, elongated, fusiform; the area is cribri- 

 form, contains three to five pairs of transversal slits, and is deep and flat. The 

 aperture is suborbicular, somewhat transverse, and is surrounded by a thick, promi- 

 nent peristome. The ovicell is small, salient, convex, smooth; it is always closed 

 by the operculum. There is often a large, simple, round avicularium placed on the 

 mural rim to the right or the left. 



JA0=0.12mm. . f?,?=0.35mm 



Measurements. Aperture , ., Zooecmm r _,... 



l?a=0.15mm. [Ls=0.70m 



Affinities. The prominence of its peristome characterizes this species. It 

 differs from Membraniporella planula in its costules, which are not flat, and in 

 the zooecial mural rims, which are joined to one another. 



From M ' cmbraniporella bioculata it may be distinguished by its less numerous 

 slits and by the entire absence of an oral mucro. 



Occurrence. Midwayan: One mile Avet of Fort Games, Georgia (rare); 

 Mabelvale, near Little Rock, Arkansas (rare). 



Cotypes. Cat. No. 63806, U.S.N.M. 



MEMBRANIPORELLA PLANULA, new speeies. 



Plate 5, figs. 8-10. 



Description. The zoarium is free and bilamellar. The zooecia are distinct, 

 separated by a furrow, elongated, fusiform, bordered by a thin mural rim; the 

 cribriform area is of little depth, formed of flat costules. and contains five or six 

 pairs of transverse slits. The aperture is semilunar. The two small oral avicularia 

 are symmetrically placed on the distal part of the mural rim; the pivot is indicated 

 by two lateral denticles. 



,, J7*a=0.10-0.12 mm. . [7,2=0.85 mm. 



Measurements. Aperture \ , -,_ Zooecia,, A0 



lfa=0.12 mm. 1 73=0.36 mm. 



Affinities. The micrometrical measurements are rather variable. Those given 

 above correspond to figure 9, but in figure S the zooecia are smaller. In the interior 

 (fig. 10) the measurements are from 0.95 to 1.10 mm. for the length and from 

 0.30 to 0.35 mm. for the width. 



This species differs from M embramporella cm-ssicottis in its more numerous, 

 flat costules. in the absence of a prominent peristome. and in its zooecia. which 

 are separated by a deep furrow. 



Compared with Membraniporella ulrichi it may be distinguished by its much 

 smaller and less constant oral avicularia (Lv 0.15 mm. and not 0.25 mm.) and by 

 the much larger slits of the cribriform area (0.05 mm. and not 0.01 mm.). 



Occurrence.- Midwayan (Clayton limestone) : Mabelvale, near Little Rock, 

 Arkansas (rare) ; Luverne, Crenshaw County, Alabama (rare). 



Cotypes. Cat. Nos. 63808, 63809, U.S.N.M. 



