60 



THE NAUTILUS. 



one (or a few) of these "forms" often prevail at a given 

 locality. 



A r Flat, compressed, dia. of shell less than 40 per cent of the 

 length (var. bigby en-sis). 

 bj. No rays, or rays obscure, color of epidermis brown, 

 dark. 



c r Rhomboid in shape. 



d t . Large. Estabrookianus. 

 d 2 . Smaller. Fassinans rhomboidea. 



c 2 . More ovate, tapering behind. Fassinans. 

 b 2 . Rays distinct, well developed over most of the disk. 

 Ground color of epidermis lighter. Bigbyensis. 

 a 2 . Moderately convex, dia. 40-49 per cent of length. 



Barnesiana typica. 



bj. Beaks not elevated, shape trapezoidal, rhomboid, or 

 subovate. 



c t . Dia. about 41 or 42 per cent; size small. 



d t . Shape somewhat elongate (trapezoidal); rays 

 obscure. Barnesxanus. 

 d 2 . Shape shorter (rhomboidal). 



e f . Rays obscure. Lenticxdaris. 

 e 2 . Rays present, color of epidermis lighter. 

 f t . Rays few. Meredithi. 

 f 2 . Rays numerous. Pudicus. 

 c 2 . Dia. about 45 per cent; larger. Shape subovate. 

 Rays obscure. Tellicoensis. 

 b 2 . Beaks more elevated, shape subtriangular. Dia. 46 

 per cent, with rather distinct rays. Lyoni. 

 a 3 . Much swollen, dia. over 50 per cent. Beaks elevated. 



(var. Tumescens). 



b l . Without rays. Dia. 51 per cent. Beaks much eroded. 



Crudus. 



b 2 . With rays. Dia. about 56 per cent or more. 



Cj. Dia. about 56 per cent. Radiosvs. 

 Co. Dia. about 64 per cent. Tumescens. 

 As to the geographical distribution, it should be briefly stated 

 that the swollen forms (a s ) inhabit the largest rivers; the com- 

 pressed forms (aj) are found in the headwaters, and the inter- 



