_Pollicipes_ is not only very ancient geologically (being found from the Ordovician upward), but it preserves the primitive characteristic of numerous skeletal plates, the peduncle being frequently covered with small calcareous pieces, which graduate into the larger more regularly placed scutes on the capitulum (Fig. 54). The species of this genus, many of which are among the largest Cirripedes, are widely distributed in the temperate and tropical seas, living for the most part attached to rocks and often in deep water. _P. cornucopia_ occurs off the English and Scottish coasts. The members of the genus _Scalpellum_, which is represented by exceedingly numerous species in the Cretaceous period, also possess a large number of plates on the capitulum, and often on the peduncle as well, but never so many as in _Pollicipes_. Although the arrangement of the plates varies much in the different species, we may describe a fairly typical case, that of the common _Scalpellum vulgare_ (Fig. 55, B). The valves of the capitulum are held together by the median dorsal piece called the “carina”; the other unpaired skeletal piece is the “rostrum,” in front, just below the place where the valves gape to allow the protrusion of the limbs. The paired pieces receive the names “scutum,” “tergum,” and “laterals,” and the peduncle is covered with rows of small plates. The genus _Scalpellum_ is a very large one, and is widely distributed, though at the time at which Darwin wrote only six species were known. The reason for this is to be found in the fact that the great majority of the species live at great depths, so that they remained unknown until the expeditions of the _Challenger_ and other deep-sea expeditions brought them to light. They may affix themselves, generally in considerable numbers together, on branching organisms, such as Corals, Polyzoa, and Hydroids, but often also on empty shells, rocks, and other foreign bodies. The body is colourless or of a pale flesh colour, but a colony of these animals, expanded and drooping in various attitudes from a piece of coral, gives the appearance of some graceful exotic flower. [Illustration: FIG. 55.—=A=, Complemental male of _Scalpellum peronii_, × 20; =B=, hermaphrodite individual of _S. vulgare_, × 2. _a_, Complemental males, _in situ_; _b_, rostrum. (=A=, after Gruvel; =B=, after Darwin.) ]