BRACHIOPODA. 



295 



Fig 013. Dielasma boHdens, Morton. 

 An enlargement of the umbonal portion of 

 the brachial valve; showing the slightly 

 thickened processes on eitlier sideof the 

 beak. (c.) 



encroaching upon the umbo and often becoming very oblique to the longitudinal 

 axis ; with the increase of this obliquity the deltidial plates are thickened in 

 their inner surface, which thus becomes more or less protruded. The inverted 

 sheath or collar within the foramen is highly developed and clearly shown on 

 internal casts. On the interior the dental plates are conspicuous, as in Cryp- 

 TONELLA, but they stand vertically upon the bottom of the valve, not showing 

 the convergence and often actual union occurring in that genus. 



In the brachial valve the dental sockets are quite deep and narrow, the soc- 

 ket-walls rising abruptly, though not attaining the height of the dental plates 

 of the opposite valve. They are distinctly separated from the crural plates or 

 margins of the hinge-plate, and converge toward 

 the apex where they merge into a slightly ele- 

 vated cardinal process ; the latter usually appear- 

 ing as a crescentic submarginal wall, though when 

 best preserved is seen to be composed of two 

 lateral, somewhat rounded lobes. The crural 

 plates are two divergent vertical lamellae, originating just below the cardinal 

 process, and attaining a length equal to the distance between their extremities, 

 which is about one-third the width of the valve at that point. Between these 

 plates lies the long shallow hinge-plate, which is raised but little above the 

 bottom of the valve, and is sometimes 

 actually adherent to it. This plate 

 attains its greatest width at the ex- 

 tremities of the vertical crural plates, 

 its margins converging thence ante- 

 riorly, its full length often equaling 

 one-third that of the valve. To this 

 plate are attached all the muscles of 

 the brachial valve, the scars of both 

 anterior and posterior adductors being 

 frequently clearly defined upon its surfoce. Upon comparison of this structure 

 with that of Cryptonella the homologies are at once apparent, but there is a 



Fig. '211. Dielasma elongaium, Schlotheim. 

 The interior of the umbonal region of the two valves; 

 showing the highly developed dental plates (rf), the elon- 

 gate, sessile hinge-plate with \l» muscular scars, and the 

 form and mode of attachment of the brachidium. 



(Davidson.) 



