134 
PALAEONTOLOGY OF NEW-YORK. 
another plait is developed on one side of the sinus, and a corresponding - one on the medial 
lobe ; but the five thus developed become two before reaching the beak. In the larger 
specimens of the New-York type (fig. Id), there is a fifth plait developed in the sinus, 
with six upon the corresponding medial lobe. The outer one on each side unites with the 
adjoining one about half way to the beak, thus making four plaits on the medial lobe, 
which finally unite in two before reaching the beak. The additional plaits of the medial 
lobe are developed laterally by a division of the outer one, which takes place at nearly 
regular intervals corresponding to the increasing size of the shell. The lateral plications in 
the sinus likewise disappear towards the beak, not by uniting with the adjoining ones as 
on the opposite valve, but by gradual diminution till they are lost in the surface of the shell. 
The plications on each side of the mesial lobe and sinus increase in like manner by the 
development of additional ones towards the margin of the shell ; and from seven, the 
number usual in the smallest shells, they increase to ten or eleven, the greatest observed 
number in any specimen presenting the characters here given ; the increase of lateral ones 
always keeping pace with the development of additional ones in the sinus and medial lobe. 
I have observed, in two specimens, a deviation from this rule ; for although they are 
much larger than any of those before noticed, there, are but three plaits in the sinus and 
four on the mesial lobe ; while the lateral plaits are twelve in one specimen, and thirteen 
in the other, on either side of the sinus. This curious deviation from the rule before 
observed, seems due to the non-development of medial plaits, which have been developed 
upon the sides instead of the sinus ; the total number on each valve being nearly the same 
as when the medial ones are more numerous. 
The western types begin in like manner with three plications in the sinus and four on 
the mesial lobe, while the lateral plications are almost uniformly seven ( sometimes six and 
rarely five). As the size increases, however, the medial plications do not, as a prevailing 
character, increase as in the eastern types, but remain still three on the sinus and four on 
the lobe, till the shells reach to four or five times the dimensions of the largest New-York 
specimens. At the same time, also, the prevailing number of lateral plications is seven on 
each side of the sinus or lobe ; and so long as the mesial plications remain three and four, 
so long the lateral ones are seven. As soon as there is even an appearance of a departure 
from this number on the mesial lobe and sinus, and where the rudiment of an additional 
plait is visible, we then find the lateral plaits to be nine or ten. These changes take place 
independent of the size of the shell, or of its rotundity ; all variations in the plaits of the 
sinus being accompanied by a change in the number of lateral plates. This is illustrated in 
the figures of nearly equal dimensions, where the specimen fig. 1 a has three plaits in the 
sinus and four on the mesial lobe, and seven on each side ; while fig. 1 g has a partial 
development of the fourth plait in the sinus, and an obscure rudiment of a fifth on the 
mesial lobe, and at the same time it has ten on each side of the sinus. Even irregularities 
in the development of the mesial plications are followed by irregularities in the lateral ones. 
A single specimen has two plications in the sinus and three on the mesial lobe, and also 
on one side of the slope of the sinus, as well as on the slope of the mesial lobe, a single 
