Hippurites. This genus belongs to a group of fossil shells whose characters are somewhat problematical, some conchologists referring them to the ordinary bivalves, and others to the Brachiopoda. Although _Hippurites_ have not been discovered in the British strata, I am induced to notice them in this place, in consequence of their great abundance in the Cretaceous deposits of the South of France, and in the Oolite of the Pyrenees; and also to illustrate the nature of a nearly related genus, _Sphærulites_, of which one or more species occur in the Sussex Chalk. The Hippurite is of an elongated conical form, and fixed by its base; it has internally a deep lateral channel, formed by two obtuse longitudinal ridges. The base is sometimes partitioned off by transverse septa, forming cells or cavities, as in the Euomphalus. The aperture, or opening, is closed by an operculum, or upper valve. The substance of the shell is cellular, and very thick, and when fractured much resembles that of the lamelliferous corals: the laminæ are sometimes separated into cells, or cavities, like the Spondyli. These shells often attain considerable magnitude, and in certain districts of the Pyrenees, where they abound, are called "_petrified horns_" by the inhabitants. It is remarkable, that, while in the Chalk of the South of France, Spain, Portugal, and Greece shells of this genus so prevail, as to be considered the characteristic fossils of the formation, in the North of France they are very rare, and in England have not hitherto been discovered.[357] [357] As marking the rapid progress of Palæontology in this country, it may be noticed that the _only fossil_ figured in the first edition of the Enclycopædia Britannica, in illustration of the article, "_Petrifaction_," is one of these supposed petrified horns, described by the Abbé Fortis. * * * * * Fossil Shells of the Lamellibranchia.--These are bivalve shells, the animals of which differ from the preceding class, as we have already stated, in performing respiration by means of lamellated gills. The valves are united by a strong substance, termed the ligament, which, by its elasticity, admits of the shells being opened to a considerable extent; and they are closed by powerful, short, thick muscles, called adductors. The shells of some of the genera, as the Oyster and Scallop, have but one muscle, (_monomyaria_); others, as the Cockle, or Cardium, and Venus, have two, (_dimyaria_); and by these characters the class is arranged in two groups. Monomyaria: _Bivalve Shells, with one muscular impression_. [Sidenote: FOSSIL OYSTERS.]