PELECYPODS 429 
are not easily detected. They are not uncommon in such stations at 
Mount Desert, Maine. 
M. plicatula. This species of Modiola ranges from Canada to Hat- 
teras, and inhabits the tidal waters of streams, sheltered muddy reaches 
among reeds, and tidal flats. Itisa dingy-looking shell, ornamented with 
Modiola plicatula, typical form. 
numerous radiating ribs, which are very fine upon the anterior portion 
of the shell. The epidermis is thin and of a dirty yellowish-green color. 
Within itis silvery-white. In the ventral margin there is an opening for 
the accommodation of a byssus. Length about three inches in New 
England; slightly larger at Norfolk. There is probably no muddy reach 
of land that is exposed at low tide from Mainé to the Carolinas that is not 
inhabited by M. plicatula. ; 
M. tulipa. This is the ‘‘ show shell” of the genus Modiola in American 
waters. It belongs to the Southern fauna, and, like the others of its 
genus, it moors itself to 
wood or other solid ob- 
jects by means of its 
byssus. It is of about 
the same size as the last 
species, but is perfectly 
smooth and of a bright- 
yellowish color, with 
dark rays over the cen- 
tral portion. 
M. recta. A species 
of Modiola fairly abun- 
dant on the Pacific coast’ 
from Vancouverto south- Modiola nigra. 
ern California. It has a i a ded 
long, narrow shell with a dark-brown glossy epidermis; lighter pos- 
teriorly, and with a short hairy growth. The interior is white. Length 
three to four inches. \ 
ORDER PSEUDOLAMELLIBRANCHIATA 
This third order of Pelecypoda is made up of a number of fami- 
lies in which the gills are filamentous, but in which the filaments 
