478 PALjBONTOLOOF OF NEW YORK. 



at its base, having two minute saddles, embracing the narrow, central por- 

 tion, where the septa are coincident with the walls of the siphuncle, and 

 penetrate* to at least four-fifths of the depth of the adjacent air-chamber. 

 Suture-lines distinctly impressed and clearly defined, but not wide; imbri- 

 cating at the summits of the paddles and at the bottom of the lobes. 



Siphuncle small, cylindrical ; its other features not determined. 



it about .5 mm. in thickness, rarely preserved in any of the specimens. 

 Surface marked by fine transverse striae, the characters not fully ascertained, 

 but which make a gentle curve forward, and bend backward toward the 

 periphery, often indicating the existence of a fascicular arrangement, or an 

 attenuation of stronger stria?, which produce the low, gentle undulations 

 sometimes observable in the worn and macerated fossil, especially toward the 

 umbilical margin. The sinus has not been determined. 



The internal cast is essentially smooth, with the suture-lines distinctly 

 impressed, and in certain conditions of weathering these lines become more 

 conspicuous from absorption or solution of the prominent portions of the 

 saddles and the angular basis of the lobes. The diameter of an ordinary 

 specimen is from thirty to fifty or sixty mm. The smallest specimen 

 measured has a greatest diameter of thirteen mm., but the individual does 

 not retain the chamber of habitation ; its greatest transverse diameter is ten 

 mm., showing it to be more rotund than the older specimens. The largest 

 specimen measured has a lateral diameter of about 100 mm., and several 

 others measure eighty to ninety mm. 



This species bears comparatively little critical resemblance to any of the 

 species of the New York formations, in none of which are the volutions so rotund 

 except G. plebeiformis, and in young forms of G. expansus. It has a general 

 resemblance, in the direction of the septa, to G. Patersoni; but in that one the 

 inner angular lobe is outside of the umbilical margin, the lateral saddle is 

 narrower and more elongate, and the outer lobe" is more extended and acute, 

 though holding a similar position on the volution. 



Formation and locality. In the Goniatite limestone, at Rockford, Indiana. 



