394 THE MEDALS OF CREATION. Cuar. XI. 
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 laminz 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 remark- 
able, 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.* 
FossiL SHELLS OF THE LAMELLIBRANCHTA.—These are 
bivalve shells, the animals of which differ from the pre- 
ceding class, as we have already stated, in performing respi- 
ration 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, (monomy- 
aria); others, as the Cockle, or Cardium, and Venus, have 
two, (dimyaria) ; and by these characters the class is arranged 
in two groups. 
* As marking the rapid progress of Paleontology in this country, 
it may be noticed that the only fossil figured in the first edition of 
the Enclycopzedia Britannica, in illustration of the article, “ Petrifac- 
tion,” is one of these supposed petrified horns, described by the 
Abbé Fortis. 
