106 BULLETIN 77, UNITED STATES NATIONAL MUSEUM. 



irregularly rounded with the posterior fourth slightly raised to form 

 the lunarium, which, however, is not conspicuous either at the sur- 

 face or in sections. The average aperture is 0.38 mm. in its longer 

 diameter and 0.30 mm. wide; four occur in 2 mm. Interzocecial 

 spaces solid, showing no trace of mesopores. 



Tangential sections show irregularly rounded to oval zooecia, with 

 evidence of the lunarium at their posterior end. Mesopores are 

 absent, the interzocecial spaces being filled with a solid tissue which, 

 in vertical sections, is seen to be of the usual laminated t3rpe common 

 to the Ceramoporidse. In the short, immature regions the zooecia 

 have especially thin walls. Diaphragms are wanting in both regions. 



The lamellate zoarium with its conspicuous, elongate, solid maculae 

 externally, and the solid, zooecial interspaces of laminated tissue seen 

 in sections, will serve to distinguish the present species from all other 

 Ordovician forms. Certain unplaced ceramoporoids from the Silurian 

 seem to have a similar structure, but, as they are undescribed, com- 

 parisons are unnecessary. 



Eichwald's figures of Archeopora punctata have a slight resemblance 

 to FavositeUa% punctata, and it is for the reason of their possible 

 s3nionymy that I have selected the same specific name for the latter. 

 Notes on Archeopora punctata are given on a later page of this work. 



Occurrence. — Not uncommon in the Wassalem beds (D3) at 

 Uxnorm, near Reval, Esthonia. 



Holotype.—C&t. No. 57178, U.S.N.M. 



Specimens and thin section in the collections of the British Museum. 



Genus SPATIOPORA Ulrieh. 



Spatiopora Ulrich, Journ. Cincinnati Soc. Nat. Hist., vol. 5, 1882, p. 155; vol. 6, 

 1883, p. 166.— FooRD, Contr. Micro-Pal. Cambro-Sil., 1883, p. 20.— Millee, 

 North Amer. Geol. and Pal., 1889, p. 323. — Ulrich, Geo!. Surv. Illinois, vol. 

 8, 1890, p. 381; Geol. and Nat. Hist. Surv. Minnesota, vol. 3, pt. 1, 1893, p. 

 319; Zittel's Textbook of Paleontology (Eng. ed.), 1896, p. 269.— Nickles and 

 Bassler, Bull. 173, U. S. Geol. Surv., 1900, p. 24.— Bassler, Bull. 272, 

 U. S. Geol. Surv., 1906, p. 21. — Cumings, Thirty-second Ann. Rep. Dep. Geol. 

 Nat. Res. Indiana, 1907, p. 756.— Hennig, Archiv fur Zool., vol. 4, No. 21, 

 1908, p. 12. 



The single species of this genus noted in the Russian strata is so 

 like an American form that it has not been difl^erentiated. Spatio- 

 pora, briefly defined, is a ceramoporoid genus in which the character- 

 istic wall structure is present, but mesopores and lunaria are prac- 

 tically absent. The zoarium is usually found incrusting the shells of 

 Orthocerata, and the zooecia, as a whole, show great resemblance to 

 the Trepostomata. 



Genotype. — Spatiopora aspera Ulrich. Upper Ordovician (Mays- 

 ville) of the Ohio Valley. 



