_Terga_, extremely variable in shape, placed at nearly right angles to the scuta: beyond their carinal ends (fig. 2 _b_), the capitulum presents two small prominences, which are important as indicating the position of the homologous, ear-like appendages in _C. aurita_.[39] The upper ends of the terga are imbedded in membrane, and project freely like little horns for about one third of their length: this free portion exactly answers to the projecting 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 approached; 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 chelonophilus_ of Leach, fig. 2 _c_), the terga are much smaller and flat, with both points sharp, the whole upper portion being much and abruptly attenuated, 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. [39] These have also been observed by Dr. Coates; see ‘Journal of Acad. Nat. Sci. Philadelphia,’ vol. vi, p. 134, 1829. _Carina_ (fig. 2 _a_), lying nearly parallel to the scuta, concave 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 acuminated. 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.