Hamilton Group 



187 



Thallostigma decipiens, n sp. 



Expausiona or masses ; cell-apertures polygonal, .33 mm in diame - 

 ter; when the surface is worn the angularity is slight, sometimes 

 they are circular, irregularly disposed; sometimes in contact, at other 

 times distant twice the diameter of an aperture; margins thin, ele- 

 vated; intermediate space occupied by angular pits, varying greatly in 

 size. The apertures and pits being polygonal and frequently nearly of 

 the same size, it is very difficult to distinguish them from each other, 

 the surface presenting very much the appearance of that of a Chsstetes 

 or Paleschara. 



Locality — York, N Y. 



Thallostigma subtilis, 71. sp. 

 Foliaceous expi^nsions ; cell-apertures oval or subpolygonal, occa- 

 sionally circular; length .20 mm.; irregularly disposed, distance from 

 each other varying from contact to twice the diameter of an aper- 

 ture ; margins elevated, and on some portions of the frond having a 

 very strong spine; intermediate space occupied by pits, nearly the 

 size of the apertures, with their margins equally elevated. 



Locality — Canada West 



Thallostigma plaita, n. sp 

 Expansions or masses ; apertures circular, diameter .40 mm , or 

 some fronds quite regularly distant from each other, slightly less than 

 the diameter of an aperture ; in other fronds very irregularly dis- 

 posed ; margins thick, elevated, the apertures being very prominent; 

 intermediate space with shallow angular pits; maculae from 1 to 2 

 mm. in diameter, destitute of cell-apertures 



Locality — Darien, N Y. 



Thallostigma spheroidea, n. sp. 

 Consisting of spheroidal masses formed by the accretion of success 

 ive layers of growth , cells tubular, cylindrical; septa infrequent; in- 

 tercellular tissue consisting of irregularly disposed vesicles ; cell-aner- 

 tures subpolygonal or circular, .50 mm. in diameter; margins elevated, 

 some portion of the margins of adjacent a])ertures usually in contact ; 

 frequently the intermediate angular pits occur on all sides of the aper- 

 ture, but often they occur only at the angles ; walls with spinules. 



Thallostigma triangularis. 

 Expansions or masses , cells tubular, cylindrical; apertures arched 

 or triangular, diameter .25 mm., quite regularly arranged, alternating 



