500 verrucidjE. 



by the fixed scutum and tergum is always steeper than 

 the other side : the colour is white or pale brownish, and in 

 V. nexa pale red. The surface is naked. The size is small, 

 rarely exceeding a quarter of an inch in diameter, and the 

 whole shell often appears like a mere scale on the surface of 

 attachment. The most remarkable feature in the external 

 aspect is due to the suture between the rostrum and carina, 

 which is formed by oblique, interlocking plates or folds ; as 

 all these plates continue to be added to at their extremities 

 during growth, the upper plates become longer than the 

 lower ones ; and the plates on both sides of the suture 

 together form a triangular area, with the broad end upper- 

 most, somewhat like the radius of a sessile cirripede : they 

 act, also, like a radius, for their growth serves to separate 

 these two valves, and so adds to the diameter of the shell. 

 The suture between the rostrum and fixed scutum and that 

 between the carina and fixed tergum are nearly of the same 

 nature, but the former is more conspicuous than the latter ; 

 neither are so conspicuous as that between the carina and 

 rostrum : accordingly as the right or left scutum and tergum 

 are moveable, so the suture, second in plainness, (see 

 PI. 21, fig. 1 a, and 1 d), is placed to the left or right 

 hand. The fourth suture, between the fixed scutum and 

 tergum, as view r ed externally, is straight, and so very ob- 

 scure that it has been overlooked by some authors, and the 

 shell described as consisting of only three nearly equal 

 pieces, for the fixed scutum and tergum together are about 

 equal in size to the carina or rostrum. The orifice ap- 

 proaches more nearly to an unequal-sided triangle, with 

 the apex broadly truncated, than to any other figure. 

 The operculum fits with remarkable closeness, and is sur- 

 rounded by a slight rim, formed by the edges of the four 

 other valves. 



Moveable Scutum andTergum. — The scutum (s in \b and 5) 

 is narrow and very small, barely equalling half the size of 

 the tergum, and therefore proportionally much smaller than 

 in any other cirripede ; in a very young shell, however, (of 

 V. Stromia) less than a pin's head in size, the scutum equalled 

 the tergum in size. The valve is remarkably thick ; it is 

 generally depressed down the middle ; but in V. nexa 



