SYNOPSIS OF GENERA. xix 



Coscinium, Keyserling (Petschora Land, p. 191. 1846). 

 [Type, Co.scinium cyclops, Keyserling.] 

 Zoarium forming lamellate expan.sions, celluliferous on both sides and per- 

 forated at regular intervals by oval or circular fenestrules. Cells arising from 

 a mesotheca, which is marked by arching undulations of growth and longitu- 

 dinal striaB. Margins of fenestrules non-celluliferous. Intercellular structure 

 vesiculose. 



Examples: Coscinium striaturum, pi. xxxiii, figs. 20, 21. 

 Coscinium striatum, pi. Ixiv, figs. 13-15. 



Coscinotrypa, Hall (Report of State Geologist for 1885, advance sheets. 

 Expl. pi, xxix. 1886). [Type Coscinium cribiforme, Prout.] 



Zoarium consisting of explanate fronds, celluliferous on both sides, with per- 

 forations or fenestrules at varying distances. At intervals the surface is raised 

 into angular folds or plicfitions, which continue growth, and in the same manner 

 give off similar elevations. Cells tubular, arising from a mesotheca. Aper- 

 tures trilobate, denticulated. Intercellular tissue vesiculose. Interapertural 

 space without mesopores. 



Example : Coscinotrypa cribriformis, var carinata, pi. xxix, figs. 29-35 ; pi. 

 xxxiii, figs. 22-25. 



Coscinella, n. o. [Type, Coscinella elegantula. Hall.] 



Zoarium of the same manner of growth and general characters as Coscinium 

 but distinguished by the presence of minute, angular pits or mesopores on the 

 surface of the frond between the cell apertures and around the margins of the 

 fenestrules. 



Example: Coscinella elegantula, pi. Ixiv, figs. 9-12. 



Ceramella, n. o. (Type, Ceramella scidacea. Hall.] 



Zoarium a thin foliaceous expansion, growing from a spreading base, celluli- 

 ferous on each face. Cells tubular, oblique. Peristomes elevated. Surface 

 marked by numerous sterile maculae which are usually depressed below the 

 general surface of the frond. 



Example : Ceramella scidacea, pi. Ixiv, figs. 5-8. 



Ptilodictya, Lonsdale (Murchison's Silurian System, p. 676. 1839). 

 [Type, Flustra lanceolata, Goldfuss.] 

 Zoarium pointed below, articulating into a spreading base ; above, a leaf-like 

 expansion, which is sometimes lobed at the distal extremity, celluliferous on 



