NORTH AMERICAN EARLY TERTIARY BRYOZOA. 841 



little or not salient, arranged in irregular quincunx. The mesopores are small, 

 polygonal, numerous, closed by a calcareous lamella. The ovicell is very large, 

 elliptical, as wide as the branches. 



Diameter of the peristome 0.14 mm. 



Diameter of the apertura 0.09 mm. 



J/Cfi#ut\'H'nfs. , Dimensions of the, ovicell 2.00 by 1.40 mm. 



Diameter of the mesopores 0.04 mm. 



Diameter of the branches 2.00-3.00 mm. 



Variations. The peristome is rarely visible. The zoarial cavities are com- 

 pletely closed ; they are often laterally constricted, preceded and followed by firm- 

 atopores, their dimensions and their forms are absolutely irregular, and we have 

 not yet discovered the reason for their formation. 



Affinities. This species differs from Ascosoecia promincns in its hollow 

 zoarium and in its club-shaped tubes. 



The ovicell is absolutely analogous to that of Cavea costata D'Orbigny, 1851, 

 but the mesopores are irregularly disseminated and not grouped symmetrically 

 on a double row between the peristomes. 



This species shows by the form of its ovicell that the generic distinction made 

 by Gregory between Petalopora, Kparsicavea. and Cavaria is useless. 



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

 Arkansas (very common) ; 1 mile west of Fort Gaines, Georgia (common). 



Cotypes.CsLt. No. 65262, U.S.N.M. 



Order CTENOSTOMATA Busk. 



Fossil representatives of this order of the Bryozoa are extremely rare com- 

 pared with those of the other orders. In the Recent seas the Ctenostomata are 

 likewise the least represented group of Bryozoa specifically, although some of the 

 species are quite abundant and widespread. But a single form of this order has 

 been discovered so far in the North American Early Tertiary rocks and it is too 

 poorly preserved to merit description as a well-defined species. 



In the Ctenostomata the zooecia are usually isolated and developed by budding 

 from the internodes of a distinct tubular stolon or stem. The orifice is terminal 

 and has an operculum of setae. Marsupia are wanting. The zoarium is horny or 

 membranaceous. 



All of the known Paleozoic Ctenostomata have been described by Ulrich and 

 Bassler in their Revision of the Paleozoic Bryozoa. 1 Little study has been put 

 upon the Mesozoic and Cenozoic forms but the Recent species have been the subject 

 of numerous papers. 



1 1904. Ulrich and Bassler. Smithsonian Miscellaneous Collections, vol. 45, pp. 256-294, 4 pis. 



