BRACHIOPODA. 295 



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- 



,. 1 • 1 11 i.u _u „U~ Fio. :13. IMetoma ftotiidens, Morton. 



ine as a crescentic submargmal wall, though when . , . r.v. , , ►• r 



o n ' o An enlargement of the umbonal portion of 



best preserved is seen to be composed of two the brachial vaue; snowing the sughtiy 



r r thickened processes on either side of the 



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 ^^^^^^ maasmaelongatum.ScMotbelm. 



nlatp Jirp nttarbpd all the muscles of ^•'* '»'«"<"• of "»e umbonal region of the two valves; 



piaie are aiiacnea an me musoiea ui g^„„,„jj ^^^^ ^j^^iy developed demai piates (d). the eion- 



iU„ 1 ^l,:„I „„1,,., iU^ c.^r...o nl' V.r>+Vi gate, sessile hinge-plato with its muscular scars, and the 



the brachial valve, the scars Ol both fo^m^nd mode of attachment of the Uiachidium. 



anterior and posterior adductors being (davidsost.) 



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

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



