46 
Aug. F. Foerste 
usually are wider, and, in most of the specimens, the relative width, 
compared with the length, is less. 
Owing to the presence of these shells in the lower part of the 
Eden formation, the question arises whether the forms under dis- 
cussion might be depauperate specimens of Strophomena hallie. 
It is then noted that, while the coarseness of the striae is about the 
same, there is an important difference shown by the pedicel valve. 
In Strophomena hallie, the pedicel valve is conca,ve except in the 
immediate vicinity of the beak. In Strophomena millionensis, 
however, the shell is elevated from the umbo anterior to the beak 
as far as the antero-lateral margins, resulting in two divergent 
areas of elevation bordering the very broad and shallow median 
depression already described. 
Some of the specimens of Strophomena scofieldi, from the base 
of the Trenton of Minnesota, present a low median fold on the 
brachial valve and a corresponding very shallow sinus on the pedi- 
cel valve; however, judging from a set of specimens kindly sent 
me by Prof. F. W. Sardeson, the majority of specimens do not 
show the sulcation, the latter not being in any strict sense char- 
acteristic of the species. The specimens attain a width of about 
20 mm. The pedicel valves are flattish, except near the beak. 
The brachial valves are moderately convex, though slightly flat- 
tened at the beak. The number of radiating striae is about 8 
in a width of 2 mm. 
It is not certain that Strophomena millionensis is the precursor 
of Strophomena sulcata, or of any of the later species having this 
form of shell. Specimens having this form may not have consti- 
tuted a distinct phylum within the genus Strophomena, but may 
have originated separately, at different times, from species of the 
ordinary form. Thus, Strophomena nnuata may have originated 
from Strophomena maysvillensis, and Strophomena millionensis 
may have originated from the same species which gave rise to 
Strophomena hallie. The sulcate forms of Strophomena scofieldi 
evidently originated from the ordinary, not sulcated forms of 
this species. From what species, however, could Strophomena 
sulcata have had its origin, if not from Strophomena sinuataf 
