96 The Avien'cdII Geologist. February, 1897 
to obscure the outline of muscle scars. The ventral muscle scar area is 
surrounded by strong ridges that join the bases of the dental lamellae. 
Very often there is a thickening in the cardinal angles and on it are fur- 
rows that coincide oftenest but not always with the direction of the 
externa) plication. 
The outer surface has angular plications transverse to whi(!h the shell 
is finely marked like O. emacerota Meek.* The jjlications on the vent- 
ral valve on the smallest specimens found have already increased to 
eleven, of which nine seem to rise from the beak and two from the car- 
dinal margin near the beak. These branch three or four times more in 
mature shells making about 80 or 120 in all. The median plication is on 
the ventral valve. On each valve the plications may be classed into 
eleven fascicles, corresponding to the eleven plications of the ventral 
valve. The offset plications in general appear alternately on the ante- 
rior and posterior sides of the older plications, but in the median and 
next two pairs of fascicles those branching toward the median line or 
forward are reduced in number by omission. 
In the median fascicle the offsets from the median plication are paired, 
but under the conditions of dwarfish growth they appear alternatingly, 
or at least the number of plications is reduced in small-sized mature 
specimens. 
The dwarfish smaller mature individuals Have proportionately larger 
visceral areas, or rather the circumference of their valves is proportion- 
ately shortened. The same are more convex than larger specimens and 
in connection with the greater convexity there are variations in the 
crura and in the plication. The whole number of variations in this spe- 
cies can therefore be approximately referred to one, viz., the variation 
in proportionate length of mesial circumference, and of this variation 
the described variation in plication of the valves is the clearest, most 
trustworthy index. In discussing the branching of the plications we 
have all the essential variations of this species indirectly taken into 
account. 
The extent of the variation of this one feature is as follows. The pri- 
mary plications and their branches give off branches alternatingly, ex- 
cept the median one which sends off two or three pairs, and except that 
secondary branches rarely give paired branches also. This stage is seen 
in large, symmetrical, flat specimens, while variation from it obtains to 
the extent that in small, convex, senile shells alternating branches only 
are given off by the median plication also, and farther that no branch 
may appear between the median and its first offset so that they are prac- 
tically paired for the part of their length. The relative time at which 
drawfish growth was induced is inconstant, but in cases where it was 
very early, the pairing of the first offset with the median plication is all 
but complete. Of course all offset plications are retarded by the relative 
shortening of the shell's circumference, but one sees it best in the med- 
ian fascicle where paired, nearly paired, nearly alternating, to alternat- 
ing offsets, to nearly paired offset and median plication obtains. The 
bilateral symmetry of the shells may be cited as the probable reason why 
*See Pal. Ohio vol. i, pi. 8, fig 2g and 3d. 
