408 SYSTEMATIC PALEONTOLOGY 



less, and often much thicker, the extremes observed being shown in the 

 illustrations; surface of wall with delicate transverse striae, usually 

 sharply rounded or angular in the middle, but when very wide the median 

 line is depressed. Numerous thin spines project from the walls into the 

 apertures but they are usually confined to the posterior half or two-thirds 

 of the opening. 



This extremely abundant and despite its variations, really very con- 

 stant species apparently belongs to the group of M. membranacea, which 

 is characterized by the absence of both avicularia and ovicells. It has 

 close relations to several European Tertiary and to certain living species, 

 but does not appear to be identical with any of the described fonns. 

 Whatever rank it may be given when the Membraniporid^ are finally 

 revised, its importance as a highly characteristic species of the American 

 Miocene demands recognition. There is no associated species with which 

 it might be confounded. 



Canu ' erects a new genus for the reception of M. memhranacea L, with 

 which as above indicated we believe the affinities of M. ohlongula lie. We 

 have not thought it wise, however, to employ the new genus until our 

 studies of the whole family arrive at a stage where we have confidence in 

 the generic grouping. 



Occurrence. — Choptank Formation. Jones Wharf, Peach Blossom 

 Creek, Greensboro, Dover Bridge, Governor Run. Calvert Formation. 

 Plum Point, Reed's, Chesapeake Beach. 



Collections. — Maryland Geological Survey, U. S. National Museum. 



Membranipora fossultfera n. sp. 



Plate CX, Fig. 1. 



Description. — Zoarium forming a thin expansion upon foreign bodies. 

 Zocecia oblong, subquadrate, sometimes obscurely hexagonal, arranged 

 in regular longitudinal and diagonally intersecting rows, with about 11 

 in 5 mm., measuring lengthwise, 9 to 10 in 3 mm., diagonally, and 11 to 

 13 of the longitudinal rows in 3 mm. Opesium elongate oval, generally 

 about twice as long as wide. Walls nearly always a little less than half 



' Bull. Soc. Geol. de France, .3rd ser., xxTiii, p. 348, where the spelling is NistcMna 

 and p. 3b0 as NicMina although the genus is presumably named after Nitsche. 



