252 BULLETIN 77, UNITED STATES NATIONAL MUSEUM. 



termined and the latter an unequivocal Hemi^Tiragma. Nicholson, 

 in remarking upon the genus in his volume ''The Genus Monticulipora," 

 opposes the resurrection of the name. 



Through the kindness of Dr. Mikhailowski, of the University of 

 Dorpat, I have had the opportunity of examining specimens of 

 Orbipora distincta studied by Dybowski, who, in turn, as stated before, 

 had access to Eichwald's types. The Dybowski -examples are of a 

 well-marked discoid bryozoan easily recognized with sufficient 

 illustrations, and agreeing with a species which I had previously 

 identified from several horizons as a form belonging to a new genus 

 of the Batostomellidse. Based upon the internal structure, which is 

 well marked in the numerous thin sections studied, this family 

 reference is made with some confidence, although a few features, 

 such as the apparent absence of maculae, cause final placement of 

 the genus to be still a matter of doubt. 



A second species, with the same internal characters, but differing 

 in having a more massive zoariam and smaller zocecia, is not uncom- 

 mon in the Orthoceras limestone (B3) at Reval and vicinity, while 

 two well marked forms occur in the lower Asaphus limestone of the 

 island of Oeland. 



Based upon these four species, Orbipora may be defined as follows : 



Zoarium typically discoid, slightly convex above, flat and with a 

 wrinkled epitheca on the underside, or of small, rounded or more or 

 less elongated masses; composed of polygonal, frequently hexagonal, 

 zooecial tubes in close contact; maculae apparently absent, if present 

 at all, quite inconspicuous ; mesopores wanting, diaphragms extremely 

 few. Acanthopores of considerable size and number, giving a spinous 

 aspect to the surface and frequently indenting the zooecial walls. 

 Thin sections show that the structure of the walls and acanthopores is 

 consistent with that prevailing in the Batostomellidse. The walls of 

 adjoining zocecia are fused together and appear as one amalgamated 

 structure instead of two distinct bands separated by a dark divisional 

 line as in other families. The acanthopores likewise have the clear 

 central structure so well marked in Lioclema and Stenopora. An 

 exceptionally well preserved specimen from the Chasmops limestone 

 on the island of Oeland shows that the acanthopore has a single row 

 of small pores or granules encircHng the inner clear structure. The 

 nature of these pores, as indeed of the acanthopore itself, is unknown, 

 but these different structures certainly had some function. 



Aside from its batostomelloid minute structure, the principal 

 generic features of Orbipora are its massive growth, absence of meso- 

 pores, few diaphragms, and ill-developed maculsB, This generic type 

 is not yet known in American strata. 



-■ Genotype. — Orbipora distincta Eichwald. Middle Ordovician of 

 Russia and Sweden. 



