168 BULLETIN 11, UlsriTED STATES NATIONAL MUSEUM. 



when I had the opportunity of studying Hall's original types and of 

 determining the generic characters to be as follows : Zoarium a sub- 

 circular, unilaminar expansion; zocecia prostrate; elongate-subrhom- 

 boidal, with erect, subtubular vestibules; apertures rounded with 

 peristome much elevated on the posterior side; interspaces depressed; 

 surface traversed by slender, bifurcating ridges which thin sections 

 show to be in reality closed canals. 



This peculiar type of structure is represented by a single species, 

 which, however, from its specific peculiarities is easily recognized. 



Genotype. — Lichenalia concentrica Hall. Silurian of North America 

 and Russia. 



LICHENALIA CONCENTRICA HaU. 



Text fig. 84. 



Lichenalia concentrica Hall, Nat. Hist. New York, Pal., vol. 2, 1852, p. 171, pi. 

 40E, figs. 5a-^.— Bassler, Bull. 292, U. S. Geol. Surv., 1906, p. 61, pi. 22, 

 figs. 1-6; pi. 26, figs. 7-10. 



As this is the genot3rpe and only valid species of the genus, the 

 specific characters are as given above. The thin, unilaminate zoaria, 

 large zocecia (averaging 0.5 mm. in their longer diameter), and the 

 meandering surface canals, are characters making the species easily 

 recognized. None of the Borkholm specimens shows any portion 

 but the epithecated base, because in breaking the limestone this 

 portion, being smooth, parts most readily. These epithecated 

 expansions differ in nowise from the same portion of other explanate 

 bryozoans, so that specimens must be either sectioned or smoothed 

 down and etched to get a view of the internal and surface characters 

 before they can be recognized specifically. Figure 84 a represents 

 the usual fragmentary specimen characteristic of the Borkholm, while 

 figures 84 & to ^ illustrate the internal structure of American speci- 

 mens in some detail. The microscopic structure is so unusual that 

 there is little danger of misidentifying the species. 



Occurrence. — ^A characteristic species of the early Niagaran rocks 

 in America; common in the Borkholm limestone (F2) at Borkholm, 

 Esthonia. 



Plesiotype.—Csit. No. 57261, U.S.N.M. 



Represented in the collections of the British Museum by speci- 

 mens from the Borkholm limestone, Borkholm. 



Family PHYLLOPORINID^ Ulrich. 



Zoarium branching; branches free or anastomosing, celluliferous 

 on one side only, the other side striated; zocecia more or less tubular, 

 often with diaphragms; hemisepta wanting. 



Both of the two genera referred to this family are represented in 

 the Russian and Swedish deposits. 



