Genera of the I^orth American Palaeozoic Bryozoa. 583 



Ptychonema, Hall. 

 (Pal. N. Y., Yol. YI, p. U, PI. IX, figs. 12-17.) 

 (PlatelT, fig. 9.) 

 Type, Ptychonema tabulatum, Hall. 



Zoarium forming spheroidal or hemispheric masses or ramose ; 

 cells polygonal ; walls very thin, strongly and regularly corru- 

 gated, not wrinkled; the corrugations forming nodes at the 

 angles ; diaphragms entirely wanting in the typical species. 



This genus may be distinguished by the strong and regular 

 corrugations of the cell walls, a feature which must not be con- 

 founded with the transverse wrinkling of the cell walls of some 

 other forms. 

 Both Monotrypa and Ptychonema are undoubtedly coelenterate 



corals. 



Stenopora, Lonsdale. 



(Appendix to Darwin's Yolcanic Islands, p. 161. ISM.) 



Type, Stenoj)ora ovata, Lonsdale. 



I have not seen a specimen of this genus, but Waagen's 

 diagnosis is as follows : 



" Zoarium incrusting, arborescent ; foliaceous or hemispherical ; 

 fixed by its base to foreign bodies ; composed of tubular cells, 

 which are nearly vertical at the center of the zoarium, and 

 radiate thence on all sides toward the surface. The cells are 

 polygonal in the center of the zoarium, with thin walls, and very 

 closely packed together ; in their radiating peripheral part they 

 become cylindrical, and their walls show regular transverse annu- 

 lar thickenings, which occur in the same manner and at equal 

 distances in a great number of adjoining tubes. These thickenings 

 alternate on the inside of the cells With periodical contractions 

 of the walls ; a vertical section of the walls being moniliform. 

 There are other thickenings which extend longitudinally in the 

 walls as in the MoNTiooLiPORiDiE. They are the thickenings which 

 precede gemmation, and project on the surface of the fronds as 

 little spines between the single cell apertures. The cell apertures 

 are partly quite open, partly quite closed by a kind of hemispheric 

 lid. They are of unequal size. Tabulae are present at very 



