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 
valves in position, and situated invariably over the dorsal edges close behind 
or under the beaks. But rarely preserved in fossil shells. 
Internal ligament or 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 VID), is found in Nucula, Pecten and many other types of the 
secondary and more recent rocks, but is rather rare among paleozoic species. 
° 
YS, 
Fig. 36. Illustrating Hinge Types, Muscles and Pallial Impressions. 
Land 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 Vanuaxemia hayniana Safford, sp., from the upper Trenton limestone of 
central Kentucky; a, area; c. ¢., cardinal teeth; J. ¢., 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 Byssonychia 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 Lyrodesma 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 line (s), and an internal ligament pit (c); 
Lutraria elliptica Roissy, Pliocene, Rhodus (one-half nat. size). The outline of this shell is to be noted in 
connection with Fig. 34, I and I1. 
: VIII, undetermined species of Clidoporus, showing clavicle (cl. ). 
Hinge teeth: This term applies to the teeth in general, but more especially when 
these are numerous and subequal, as in Tellinomya. 
