One of the most ancient Cirripedes known is Turrilepas, which occurs in the Silurian deposits of England, but it is also known from earlier deposits, while undoubted Cirripedes have been found in the Cambrian of North America. The body of Turrilepas is enclosed in imbricating plates, as shown in Fig. [53], A.
In Archaeolepas of the Upper Jurassic (Lithographic slates of Bavaria) the arrangement of scutes typical of the Lepadidae is foreshadowed, but the whole of the peduncle is protected by rows of plates (Fig. [53], B), as in Turrilepas.
The above-mentioned genera did not survive into the Cretaceous period, their places being taken by the genera Pollicipes and Scalpellum, which first appeared in the Silurian and persist to the present time, the older and more primitive Pollicipes being represented by about half a dozen living species, while the species of Scalpellum are exceedingly numerous.
Fam. 1. Polyaspidae.—This family includes the three genera, Pollicipes, Scalpellum, and Lithotrya.
Fig. [54].—Pollicipes mitella, × 1. (After Darwin.)
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.