40 [Assembly 



ceolate or hastate; and from the centre of the concave margin be- 

 tween the crura issues a slender process, which penetrates into the 

 cavity of the ventral valve. This process, with the two first described 

 as extending into the cavity of the ventral valve, sometimes reach 

 nearly to the inner side of the shell, the three gradually converging 

 to the extremities which are near together. This peculiar apparatus 

 is not attached to any median septum ; and the plate is left without 

 any support, except from the slender crura. 



The cardinal process at the base of the crura is often much 

 thickened, and sometimes extends forward into the shell much more 

 than in others; and when it becomes thickened in old shells, is 

 often distinctly marked by two grooves upon its summit. Behind 

 this process and between it and the beak, there is a distinct round 

 foramen communicating beneath with the interior cavity of the 

 valve. The points for the attachment of adductor muscles in the 

 dorsal valve are double. 



The internal structure described lias been faljy determined in two species, 

 and partially seen in others. In one species, twenty or more individuals have 

 shown it, with some slight variations in the form of the longitudinal plate, 

 as illustrated in the figures on Plate evil, Paleeontology N.Y. Vol. iii. 



The shells of this genus are usually oval or ovate in outline, and often 

 very ventricose ; some species varying greatly in their difierent stages of 

 growth. At present, I know of but a single exception to the form men- 

 tioned. 



In the greater number of species, the lateral margins of the shell are 

 bent abruptly inwards, often at right angles, or still more abruptly, so as 

 to leave an angular groove along the margin of the united valves. This 

 character is sometimes seen in the young shell, while often it appears only 

 in the more. advanced stages of growth. The species vary greatly in size, 

 ranging from the smallest to nearly that of the largest brachiopod of this 

 general form in the palseozoic rocks. The largest specimens figured have a 

 length of three inches, and I have fragments of others which have been, 

 mueli larger. 



The geological range of the genus, as at present known, is from the 

 upper part of the Lower Helderberg group, through the Oriskany sandstone, 

 and into the Upper Helderberg limestones. 



In its geographical range, it is known from Graspe in Canada East, to 

 Virginia and Tennessee (and probably occurs in Alabama), and westerly 

 from New-York through Canada West, Mackinac island, Ohio, Illinois and 

 Missouri. 



The accompanying figures illustrate the characteristics of the genus^as 

 shown in two species. 



