270 BRITISH SILURIAN BRACHIOPODA. 



the width of the shell, while in other cases, and particularly in the older and more 

 gibbous specimens, the cardinal line is less than the width of the shell, and its extremities 

 are rounded. There are, somewhat rarely, exceptions to the above observation, where the 

 cardinal line, in old individuals, still extends into small acute ears. The number of 

 plications is very variable also ; but there is usually a relation between the number of 

 those on the sinus and medial lobe and those on either side. The greater or less number 

 of these plications, however, has no reference to the form of the shell. This species, like 

 many others of the Brachiopoda, is influenced by local circumstances, and, in its wide 

 geographical distribution, presents varieties of form, or types peculiar to different localities, 

 dependent apparently upon the condition of the ancient ocean-bed. In the dark carbonaceous 

 Silurian limestones of New York it is almost always small, while in the lighter coloured 

 calcareous mud or shale of the west it reaches a much greater size, and presents a greater 

 variety of form. The eastern type of this species presents the following characters in its 

 different stages of growth and development : — 



" In the young shell there first appears three plaits in the sinus of the dorsal valve, 

 with four on the corresponding medial lobe of the ventral valve ; these four becoming two 

 about one half or two thirds of the distance from the base to the beak. As the shell 

 grows older 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 there is a fifth plait developed in the sinus, 

 wdth 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 aud 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 



" 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, 

 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 foin- or five times the dimensions of the largest New York 

 specimens. At the same time, also, the prevailing number of lateral phcations is seven 

 on each side of the sinus or lobe, and so long as the mesial plications remain three and 



