120 BTXLLETIN 125, UNITED STATES NATIONAL MUSEUM. 



developed, the tremopores appear large (forma praeciliata). A specimen from 

 Carteret County, North Carolina, is very suggestive. The incrusting portion 

 shows normal zooecia with granulations and tremopores (fig. 1) ; a bilamellar 

 expansion shows, on the contrary, zooecia without granulations but with large 

 tremopores (fig. 2). A specimen from Santa Barbara, California, has very small 

 avicularia as in Microporella umbonata, but nevertheless its dimensions are ab- 

 solutely identical with Microporella ciliata. 



T,f . A . (Aa = 0.04-0.06 mm. „ . [is = 0.50 mm. 



Measurements. — Apertura , _ ,„ . ,„ Zooecia , „ „„ 



^ 1(0. = 0.10-0.12 mm. [te = 0.30mm. 



The coast of California is the Elysium of the genus Microporella, for a wealth 

 of species occurs there. They are very closely related and their determination 

 is not made without difficulty because of their polymorphism. It is necessary 

 especially to take into consideration the reciprocal place occupied by the avicula- 

 rium and the ascopore; the micrometric measurements are of value only in extreme 

 cases. 



Occurrence. — Miocene (Choctawhatchee marl) : Jackson Bluff, Ocklockonee 

 River, 2.5 miles southwest of Tallahassee, Florida (rare). Miocene (St. Mary's 

 formation): Cove Point, Maryland (rare). Miocene; Kuhns, Carteret Coimty, 

 North Carolina (rare). Miocene (Choptank formation); Jones Wharf, Maryland 

 (rare) . Pleistocene : Santa Monica (rare) ; Dead Mans Island, off San Pedro (rare) ; 

 and Santa Barbara, California (rare). 



Geological dtstribution. — Stampian of Germany (Schreiber) ; Chatian of Ger- 

 many (Reuss); Miocene of Austraha (Waters), of Tunis (Canu) ; Burdigalian of 

 Gard in France (Canu collection) ; Helvetian of Italy (Seguenza Neviani), of France 

 (Canu collection); Tortonian of Italy (Seguenza), of Austria-Hungary (Reuss); 

 Sahelian of Oran (Canu collection); Zanclean of Italy (Seguenza); PUocene of 

 New Zealand (Waters); Plaisancian of Algeria (Canu), of Italy (Manzoni), of 

 England (Busk) ; Astian of Italy (Seguenza) ; Sicilian of Italy (Seguenza, Manzoni, 

 Neviani), of Rhodes (Manzoni); Quaternary of Italy (Waters, Seguenza, Neviani, 

 De Stef ani) . 



Habitat. — Cosmopolitan. 



Plesiotypes.—C&t. Nos. G8584-G8590, U.S.N.M. 



MICROPORELLA HEXAGONA, new species. 



Plate 20, figs. 14, 15. 



Description.~The zoarium incrusts shells. The zooecia are Uttle distinct 

 separated by a furrow, short, hexagonal,conyex, gibbose; the frontal is convex,' 

 irregular, and formed of a tremocyst with large pores detachable from a subjacent, 

 finely perforated olocyst. The apertura is deeply buried, semilunar; the peri- 

 stome is somewhat salient, very thin, and bears four to six large, hollow spines. 

 The ovicell is little salient and is covered by an incomplete tremocyst. The asco- 

 pore is large and is surrounded by a very salient peristome. There are two small 

 avicularia or a single one very large and salient. 



Measurements.-Apertuvol f^ = ^f J ^^- Zooecia( ^" = "^-^^^-^^ °^^- 



lto = 0.12-0.15mm. [Z2 = 0.45mm. 



