ORISKANY SANDSTONE. 
455 
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 cvn. 
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 number of species, 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 brachiopod of this general form in the palaeozoic rocks. The largest specimens 
