148 CONCHODERMA VIRGATA. 



jecting portion, bounded by the two occludent margins, 

 in the terga of Lepas. The freely projecting portion is 

 generally curled inwards, and the carinal portion more 

 or less outwards, — the form of the letter S being thus ap- 

 proached ; but the curvatures are not exactly in the same 

 plane. The whole valve is generally of nearly equal width 

 throughout, the carinal part being a very little (but in some 

 specimens considerably) wider; internally, it is deeply 

 concave ; both points generally are blunt and rounded. In 

 some rare varieties (Cineras c/ielonop/iilus of Leach, fig. 2 c), 

 the terga are much smaller and flat, with both points sharp, 

 the whole upper portion being much and abruptly at- 

 tenuated, and internally, without a trace of a concavity. 

 Generally, the terga are about two thirds of the length 

 of the scuta, rarely only half their length ; generally, 

 they are separated from the apices of the scuta by about 

 their own length, rarely by twice their own length. 

 Generally, the terga are shorter than the carina, but 

 sometimes a very little longer than it : generally they are 

 distant by one third or one fourth of their own length 

 from the apex of the carina, rarely by their entire length. 



Carina (fig. 2 a), lying nearly parallel to the scuta, con- 

 cave within, very slightly bowed, of nearly the same width 

 throughout, but with the lower third beneath the umbo, 

 generally a trace wider than the upper part. Length, 

 variable, generally rather longer (sometimes by even one 

 third of its own length) than the scuta, but sometimes 

 equalling only three fourths of the length of the scuta ; 

 generally longer than the terga. Upper and lower points 

 rounded ; in rare varieties, both ends are sharply acumi- 

 nated. The carina and terga are generally most acuminated 

 where they are smallest and least perfectly calcified ; and 

 consequently, in this same state, the valves stand furthest 

 apart. 



Peduncle, flattened, gradually widening as it joins the 

 capitulum, to which it is generally about equal in length, 

 or a little longer. 



Filamentary Appendages. — Six on each side (PI. IX, fig. 4), 



