GENUS VERRUCA. 501 



this part is longitudinally ribbed. The occludent margin 

 is curved. On the tergal margin there are two articular 

 ridges (with a deepish furrow between them), of which the 

 upper one (' in s, in fig. 1 b, and 5) extends from the apex 

 about half-way down the valve ; and the other, or lower 

 articular ridge (" in s), generally runs down nearly to the 

 basal margin : an angle, running from the apex to the 

 basi-tergal corner of the valve, appears like a third articular 

 ridge, but cannot properly be considered such. The above 

 two articular ridges interfold with analogous ones on the 

 scutal margin of the tergum, and so lock the valves together. 

 On the under side (fig. If), the surface is bounded along 

 the occludent margin by a slight rim : there is generally 

 a very slight depression for the adductor muscle; but in 

 V. Spengleri there is a straight, short, sharp (PI. 21, fig. 2), 

 prominent adductor ridge. 



The moveable tergum is broad and rhomboidal. Externally 

 a prominent axial ridge ('" in t, in fig. 1 b, &c), which 

 widens downwards, runs from the apex of the valve to the 

 basal point, and there projecting slightly, causes the scutum 

 to be indented ; this indentation on the scutum appears like 

 a third articular ridge, lying beneath ("), s, in fig. 1 b, &c. 

 Above the lower and axial ridge, on the scutal margin of 

 the tergum, there is a middle articular ridge, which locks in, 

 between the lower (") and upper articular ridges (') of the 

 scutum (s). Again above the middle ridge there is au 

 upper and third articular ridge 0, which is either quite dis- 

 tinct, as in fig. 5, t, or more commonly is formed by the 

 occludent margin of the valve, as in t, fig. 1 b. The 

 broad extremity of this upper articular ridge is often pro- 

 duced into a slight projection, or shoulder, and this always 

 underlies the scutum, of which the under and upper sur- 

 face is indented or furrowed (see fig. If), in order to re- 

 ceive this shoulder. The upper articular ridge of the 

 scutum (', s, 1 b) locks in between the upper articular ridge 

 or occludent margin (', t), and the middle ridge (", t) of 

 the tergum. 



Hence, altogether, there are three articular ridges on the 

 scutal margin of the tergum, the occludent margin being gene- 

 rally counted as one ; whereas, on the tergal margin of the 



