BRACHIOPODA. 



479 



Terminology.] 



Anterior, posterior and ventral slopes are self-explanatory terms. 



External ligament:. An elastic, horny band, of variable length, serving to hold the 

 vrflves in position, and situated invariably over the dorsal edges close behind 

 or under the beaks. But rarely preserved in fossil shells. 



Internal ligament ot cartilage: This is generally of cartilaginous consistency, and 

 often but a modification or extension of the external ligament. In the latter 

 case it lies along the posterior inner border of the hinge, where its presence 

 may be indicated by linear thickened supports which, in casts of the interior, 

 may sometimes be confounded with impressions of lateral teeth, (Whitella). 

 A true internal cartilage, usually occupying a small pit beneath the beaks 

 (Fig. 36, III and VII), is found in Nucula, Peden and many other types of the 

 secondary and more recent rocks, but is rather rare among paleozoic species. 



vw 



Fig. 36. Illustrating Hinge Types, Muscles and Pallial Impressions. 



I and II, Interior of a right valve of Lyrodesma major Ulrich, of the natural size, and the rostral 

 portion of the valve x 2; upper beds of the Cincinnati group of Ohio. 



III, hinge of a species of Nuculana, showing internal cartilage pit at c. 



IV, interior of a left valve of Vanuxemia haynidka Safford, sp., from the upper Trenton limestone of 

 central Kentucky; a, area; c. t., cardinal teeth; 1. 1., posterior lateral teeth; a. a., anterior adductor, and 

 p. a., posterior adductor impression; p., pallial line. 



V, cast of the interior of a left valve of an unnamed variety of Byssoyiychia radiata Hall, sp. 

 (Ambonychia bellistriata Miller and others, not Hall, 1847,) from the lower beds of the Cincinnati group of 

 Ohio. In this specimen the posterior adductor impression (p. a.) and the pallial line are usually distinct. 



VI, a sharply defined cast of the interior of I/yrodesma major Ulrich (see also I and II), showing the 

 muscular impressions in a very satisfactory manner; a. a., anterior adductors; p. a., posterior adductors; 

 p. m., two pairs of pedal muscles. 



VII, interior of a shell with a strongly sinuate pallial liue(s), and an internal ligament pit (c); 

 Lutraria elliptica Eoissy. Pliocene, Ehodus (one-half nat. size). The outline of this shell is to be noted in 

 connection with Fig. 34, 1 and II. 



VIII, undetermined species of Clidoporus, showing clavicle (cl.J. 



Hinge teeth: This term applies to the teeth in general, but more especially when 

 these are numerous and subequal, as in Tellinomya. 



