f1. Rays few. Meredithi.
f2. Rays numerous. Pudicus.
c2. Dia. about 45 per cent; larger. Shape subovate. Rays obscure. Tellicoensis.
b2. Beaks more elevated, shape subtriangular. Dia. 46 per cent, with rather distinct rays. Lyoni.
a3. Much swollen, dia. over 50 per cent. Beaks elevated. (var. Tumescens).
b1. Without rays. Dia. 51 per cent. Beaks much eroded. Crudus.
b2. With rays. Dia. about 56 per cent or more.
c1. Dia. about 56 per cent. Radiosus.
c2. Dia. about 64 per cent. Tumescens.
As to the geographical distribution, it should be briefly stated that the swollen forms (a3) inhabit the largest rivers; the compressed forms (a1) are found in the headwaters, and the intermediate forms (a2) belong to the streams of moderate size. Intergrades are frequent.