136 
MUSEUM BULLETIN NO. 2. 
the one-plication shell has its one plication, and the additional 
plications, when formed by implantation, are almost always 
added outwardly towards the side of the sinus. 
Third, — Among the Fifth Avenue specimens, which come 
from a higher horizon, there are none having no plications 
or one plication in the sinus. There are so few specimens 
from here, however, that perhaps it is hardly sufficient evidence 
to have much weight. 
Hence, it seems only reasonable to infer that those groups 
with more plications, as groups, have their origins in the groups 
with fewer plications. In the stage of development of the 
Prasopora beds, and the Anticosti specimens, the groups with 
two and three plications apparently predominated, though in 
the former those with none and one still existed, and those with 
four and five had evidently made good headway. 
Relationship of Horizon and Plication Development . — The 126 
specimens measured and examined were collected at three 
different horizons. From the lowest, the base of the Trenton 
at Indian Lorette, there are only two specimens, both considerably 
worn, one having one plication in the sinus, the other two. 
From the Prasopora beds around Ottawa and Hull there are 
96 specimens, and from Fifth avenue, Ottawa, a still 
higher horizon, there are 28 specimens. The Indian Lorette 
specimens are too few in number to furnish any grounds for 
comparison, but there are two interesting features shown by 
comparing the specimens from the Prasopora beds and those 
from Fifth avenue. The first has already been mentioned — 
the absence from the higher horizon of any specimens having no 
plication or one plication. The second is the much greater 
number of plications on the sides of the shells from Fifth 
avenue. Comparing similar groups of those from the Prasopora 
beds and those from Fifth avenue it is found that the 
Fifth Avenue specimens average smaller in length, width, 
height, etc., though the average ratio of length to height is the 
same, implying that in size at least they had reached their 
fullest development. The Anticosti species, like the Fifth 
avenue specimens, shows a greater number of plications upon the 
sides, but unlike them is in every dimension about double the 
