Terminology. ■] 



BEAOHIOPODA. 



477 



concentric, striation (growth lines) of the surface. But when the mantle edges are 

 undulating or dentate the concentric growth lines are crossed by radiating striae 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. 3,5. I and II, right valves of Ischtrodonta (?) otalis Ulrich and Mathbhia kugosa Ulrich, 

 lettered and divided by lines to illustrate the section on outline and dimensions. A-B, length; C-D, 

 anterior hight; B-F, posterior hight; a b, dorsal or cardinal margin; c d, anterior end and margin; e /, 

 basal or ventral margin; g h, posterior end and margin. 



III, antero-cardinal vievy of a small specimen of Guneamya curta Whitfield, from the upper part of 

 the Cincinnati group of Ohio; u, umbones and beginning of umbonal ridges; e, escutcheon; I, lunule. 



IV, the two valves of an undescribed species of Orthodesma lying open in the shale and showing the 

 ligament at I; middle beds of the Cincinnati group at Cincinnati. 



V, dorsal view of an entire cast of the interior of Ghcenodomus 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 

 tayssal 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. 



