ORISKANY SANDSTONE. 45S 



then one division of each, diverging into the centre of the ventral valve, 

 terminate in acute points. On the other side the divisions extend nearly 

 at right angles to the axis of the shell, into the cavity of the dorsal valve ; 

 and thence bending abruptly forward and gradually converging, terminate 

 above the centre of the shell in a thin flattened or longitudinally concave 

 plate, which, at its remote extremity, ends in an acute point, the whole 

 being lanceolate or hastate ; and from the centre of the concave margin 

 between 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 ex- 

 tremities which are near together. This peculiar apparatus is not attached 

 to any median septum ; and the broad 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 attachment of the ad- 

 ductor muscles in the dorsal valve are double. 



The internal structure described has been fully 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 cvii. 



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

 tricose; some species varying greatly in their different stages of growth. At present, 

 I know of but a single exception to the form mentioned. 



In the greater niuuber of sjiecies, the lateral margins of the shell are bent abrupt- 

 ly 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 brachiopodof this general form in the palaeozoic rocks. The largest specimens 



