132 
MUSEUM BULLETIN NO. 2 . 
avenue, Ottawa. For comparison a study was made of eleven 
typical specimens of Parastrophia reversa (Billings) 1 from the 
Lorraine formation of the Island of Anticosti. Billings later 
stated that this species was simply a variety of Parastrophia 
hemiplicata Hall. 2 The most striking differences are their size, 
and their greater average gibbosity, as indicated by the ratio 
of height and length. The mesial fold and corresponding 
sinus also are relatively less prominent than those of Parastrop- 
hia hemiplicata , Hall. 
For the majority of the specimens the writer is indebted to 
the kindness of Mr. J. E. Narraway of Ottawa, by whom all the 
Fifth Avenue specimens were loaned, as well as a large 
number from the Prasopora beds. The rest are in the collections 
of the Geological Survey of Canada. 
The data taken for study are: length of shell; width of shell, 
measured at the widest part; height; width of sinus, measured 
at the anterior margin; number of plications on each side of the 
sinus on the pedicle valve; and the total number of plications. 
As stated above, the number of plications or striae in the sinus 
is the basis for division into groups. 
Each shell in its nepionic stage is without a plication, even 
the sinus not being indicated; as' it grows, anteriorly, it forms 
the sinus and fold, later come the plications in the sinus and on 
the sides. Would every young shell, if undisturbed in its growth, 
continue to develop more and more plications until it reached 
the maximum number, the five-plication form? Or, does each 
shell start out with an inherent tendency to form one, two, 
three, or four plications in the sinus, as the case may be? From 
the specimens examined and measured it would seem that the 
latter alternative is the more probable. 
Inherent Tendency to a Definite Number of Plications. First. — 
Each group of no-plications in the sinus, one, two, or three 
plications, etc., has adult representatives. Figure 1, Plate IV, 
for instance, probably represents an adult, or nearly adult 
specimen, with no plications in the sinus or on the sides, merely 
the two ridges which bound the sinus. There is no indication of 
iGeol. Surv. of Can. Report of Progasss for 1856, p. 295, 1857. 
2Can. Jour. IV, p. 316, 1859. 
