FOSSILS OF THE CARBONIFEROUS. 221 
variable, and also specifically identical with the widely distributed R. radi- 
ais. All the specimens agree in general form, the presence of a small 
area and a vertically ribbed, punctate shell. In the number and strength 
of the radiating costa, however, the greatest difference is shown. I now 
have before me specimens with 7, 10, 12, 13, 17, 22, and 25 costee on each 
valve. With the two extremes to base a separation on both Retzia Mormoni 
Marcou, and R. radialis, var. grandicosta Davidson, appear as good species— 
one characterized by a few angular radiating ribs, and the other by numer- 
ous rounded radiating ribs. With the intermediate links to connect the 
shell with seven ribs and that of twenty-five and, also, any variations of 
form that occur among many examples, we are necessitated to unite all 
under one species, and to refer it to Retzia radialis of Phillips, as Davidson 
has shown that that species has the same range of variation in the number 
and character of the radiating ribs, and a comparison of the American 
specimens with his figures and descriptions leaves little, if any, doubt of 
the specific identity of the two. 
In the White Pine shales of the Upper Devonian of the White Pine 
Mining District, Nevada, a shell occurs that is identical with the more finely- 
ribbed examples from the Lower Carboniferous limestone of the Pancake 
Mountains (situated between White Pine and Eureka) and those occurring 
at relatively the same horizon near Santa Fé, New Mexico. 
As far as our observations go, the finely-ribbed variety appears first in 
the Upper Devonian, and the more coarsely ribbed in the Middle Carbon- 
iferous in association with the intermediate forms, uniting them with the 
finely-ribbed variety. 
The coarsely-ribbed variety described by Davidson, from the Punjab, 
India, is represented by the Nevada shell bearing seven plications, and the 
more finely-ribbed form from Russia is the usual variety from England, 
Belgium, and America. 
Dr. Kayser has placed the specimens representing the species in China 
under £. compressa Meek, a variety of R. radialis Phillips. All the differ- 
ences mentioned by Meek as characterizing R. compressa, as distinct from 
R. punctulifera, are shown in a good series of R. radialis. 
