MARYLAND GEOLOGICAL SURVEY 411 



distinguished by the smaller size and less elongate form of its zooecia. 

 The walls also are relatively thicker while the longitudinal arrangement 

 of the zooecia is more pronounced. Measuring longitudinally, 8 zocecia 

 occur in 3 mm. and transversely 12 may be counted in the same space. 



Occurrence. — Calvert Formation. Eeed's. 



Collection. — Maryland Geological Survey. 



Membranipora bifoliata n. sp. 

 Plate CXII, Figs. 2, 3, 4. 



Description. — Zoarium forming small, bifoliate, undulating fronds, 

 the two halves of which separate readily. Zooecia squarish-hexagonal on 

 the front face, oblong-quadrate on the basal or inner side where the 

 slightly curved end-walls show through the longitudinally fluted basal 

 lamina, and where the ratio of length and breadth of the under side of 

 the zocecia varies from 3 to 2 to 4 to 2, respectively ; arranged in longi- 

 tudinal and diagonally intersecting rows, of which the former are the 

 more conspicuous and regular. Measuring longitudinally about six 

 zofficia, rarely only five, occur in 3 mm.; diagonally six or seven and 

 transversely eight or nine may be counted in the same distance. Opesia 

 large, usually varying from narrow to broad-ovate, rarely subcircular. 

 Walls nearly equally thick all around, usually with a narrow median 

 furrow, and the apertural rims finely granulose or transversely striated, 

 the striae becoming stronger and the median furrow obscure with age. 

 One or two small pits, with their mouths directed obliquely forward, 

 generally present in the posterior angle or angles of each zocecium. 

 Occasionally a small node is associated with these pits or may take the 

 place of one. Ko ovicells nor avicularia observed. 



Of American fossil species the only one that may be considered as at 

 all related is the M. rimulata Ulrich, from the Eocene at Upper Marl- 

 boro, Md. In that species, however, the zooecia are more hexagonal and 

 the zoarium adnate and not erect and bifoliate. It also has large vicar- 

 ious avicularia of which no trace has been discovered in M. bifoliata. 



Three of d'Orbigny's Cretaceous Biflustras, B. emarginata, B. papy- 

 racea and B. cyclopora, the first two of which Canu identifies with 

 d'Orbigny's B. ovalis, are perhaps closely related to our species, but we 



