432 
BULLETIN OF THE UNITED STATES FISH COMMISSION. 
anterior half of the shell. The shell is thick, reticulate or striped, and sometimes provided with short 
spines; often distinctly porous. The anterior half is composed of 3 equatorial and 3 apical plates, the 
latter being continued into the horn-like process. The posterior half is composed of 3 equatorial and 
one apical plate continued into the posterior horn. The right posterior plate is continued into a 
similar horn which may remain rudimentary or be continued into a considerable process. Similarly 
the left posterior horn is usually developed, but remains small. There may be from 2 to 3, 4, and 5 
horns. Chromatophores usually present, green to yellow brown. 
Fresh and salt water. 
Ceratium tripos Ehr. Fig. 25. 
The body is somewhat triangular and bears three horns, two of which are shorter than the other 
one and slightly curved upward. 
Length, including the horns, 290/*. 
Fig. 26 . — Ceratium fusus. 
Ceratium fusus Ehr. Fig. 26. 
Synonym: Peridinium fusus Ehr. 
The animal is very elongate, due to the presence of two long horns at the extremities of the body. 
Color, yellow with chromatophores. Length 285/*; width 23/*. 
Both of these species are common in the tow 
and in the alga^ at the edge of the wharf. Both 
of them are mentioned by Peck in '93 and ’95. 
Genus AMPHIDINITJM Clap. & Lach. 
The body is ovoid to globular and usually 
much flattened dorso-ventrally. The anterior 
portion is very much reduced and is somewhat 
head-like or cap-like. The longitudinal furrow 
extends through the entire posterior body 
length and is apparently capable of widening 
and narrowing. It is probably naked (see here 
Klebs, Pouchet, Biitschli) , although Stein main- 
tained that there is a delicate cuticle-like shell. 
Chromatophores of brown or green colors pres- 
ent and usually grouped radially about a central 
amylum granule. The nucleus is posterior. 
Fresh and salt water. 
Amphidinium operculatum Clap. & Lach. 
The body is oval and flattened. The trans- 
verse furrow is at the extremity (posterior) of 
the body and the small portion, which is thus 
apparently cut off, is the cap-like or operculum- 
like structure which gives the name to the species. Klebs maintains that the two furrows are not 
connected, but in this he is certainly mistaken, provided we have the same species under consideration. 
Very common about Woods Hole. 
Length from 40 to 50//; width 30//; thickness 15//. 
