134 fAL^ONTOLOGY OF NEWiYORK. 



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 Interal ones are seven. As soon as there is even an appearance of a departure 

 from this number on the mesial lobe and siiuts, 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 

 siinis being accompanied liy 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 ])hiit in the siiuis, 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 tlie development of the mesial plications are followed by irregidarities in the lateral ones. 

 A single specimen Ikis two plimlioiis in the sinus and (liiee on the mesial lobe, and also 

 on one side of llie slope of (he sinus, as well ay on the slope of the nu'si;d lobe, a single 



