BRACHIOPODA. 143 



the shell ; it is composed of two elongate-ovate adductor scars enclosed by 

 broad and radially striated diductors. A conspicuous median septum begins in 

 the urabonal region and extends to within a very short distance of the anterior 

 edge of the valve. 



In the brachial valve the hinge-plate arises with a vertical anterior face 

 from the bottom of the shell; but just above the plane of the margins of the 

 valve it is reflected in a curve so abrupt that its upper face becomes horizontal. 

 The anterior face is concave and quadrate in outline ; the posterior face is 

 subtriangular, flat or concave, and is frequently bilobed at its extremity. In 

 profile the plate has a hook-shaped appearance; its posterior extremity being 

 elevated considerably above the beak of the valve, and when the valves are 

 in articulation, extending quite to the bottom of the umbonal cavity of the 

 pedicle-valve. The crural bases are situated on the vertical face of the plate, 

 just at the point of recurvature. The crura are slender, straight, long and 

 rod-like, having a length equal to fully one-fourth that of the shell. They 

 are attached at their tips to the inner surfaces of the primary lamellae. The 

 primary lamellae of the spiral coils are greatly incurved and their apices close 

 together ; their umbonal blades are very broad. The loop originates at about 

 one-fourth the length of the lamellae, is inclined slightly backward, the 

 lateral branches uniting directly in front of the apices of the lamellae, and 

 forming a simple straight stem, which is continued beyond the opposite edge 

 of the coil and almost to the inner surface of the pedicle-valve. The spiral 

 ribbon makes from six to ten volutions, and the cones have their altitude in 

 the transverse diameter of the shell. 



The muscular area is very narrow and elongate, the posterior adductor scars 

 enveloping the extremities of the anterior adductors. They are divided into 

 pairs by a median septum of the same extent as that of the pedicle-valve. 

 Fine racemose vascular sinuses are sometimes retained over the pallial region 

 of both valves. 



The external surface usually bears a low median sinus and fold on the 

 pedicle and brachial valves respectively. The epidermal layer of the shell is 

 usually, probably always, covered with numerous fine, short spinules; these. 



