BRACHIOPODA. 477 
Terminology.] 
concentric, striation (growth lines) of the surface. But when the mantle edges are 
undulating or dentate the concentric growth lines are crossed by radiating strize or 
plications. 
The various parts of the shell are conveniently brought out and illustrated in 
the following section on terminology. 
TERMINOLOGY. 
Outline: The designation of the various parts of the outline depends upon the 
position in which the shell is placed. I shall adopt, because it is certainly the 
most convenient if not always the most natural position, the one in which 
the beaks are placed uppermost and the hinge line nearly or quite horizontal. 
The part in front of the beaks, toward. which they are usually inclined, is 
therefore considered as the anterior end, while that behind them, often much 
the largest and widest, is the posterior. The upper edge is known as the 
cardinal or dorsal margin, while the lower is called the basal or ventral. 
Dimensions: The length as given in the following pages always expresses the distance 
between the most prominent points (extremities) on the anterior and posterior 
Fig. 35. I and II, right valves of IsSCcHYRODONTA (?) OVALIS Ulrich and MATHERIA RUGOSA Ulrich, 
lettered and divided by lines to illustrate the section on outline and dimensions. A-B, length; C-D, 
anterior hight; E-F, posterior hight; a 6, dorsal or cardinal margin; ¢ d, anterior end and margin; e f, 
basal or ventral margin; g h, posterior end and margin. 
III, antero-cardinal view of a small specimen of Cuneamya curta Whitfield, from the upper part of 
the Cincinnati group of Ohio; x, umbones and beginning of umbonal ridges; e, escutcheon; J, lunule. 
IV, the two valves of an undescribed species of Orthodesma lying open in the shale and showing the 
ligament at 7; middle beds of the Cincinnati group at Cincinnati. 
V, dorsal view of an entire cast of the interior of Chenodomus typicalis, a new genus and species 
from the upper beds of the Cincinnati group of Ohio, showing a shell gaping at both ends. 
VI, anterior view of Byssonychia radiata (Ambonychia radiata Hall), illustrating a shell with a 
byssal Opening. This specimen is from Cincinnati, Ohio, and is peculiar in having the right valve (left 
side of figure) preserved as a cast of the exterior and the left valve as a cast of the interior. In the latter 
is shown the pallial line (at p) running along the anterior side to a point under the beak. 
