EQUIVALVE. 145 



stomata of the same author, both with regard to the soft parts, 

 and their testaceous covering. This family partly answers to 

 the Purpuriferae in the system of Lamarck, and contains the 

 genera Subula, Cerithium, Melanopsis, Planaxis, Terebra, Eburna, 

 Buccinum, Harpa, Dolium, Cassidaria, Cassis, Ricinula, Cancel- 

 laria, Purpura, Concholepas. 



EOLIDES. Montf. A genus of microscopic Foraminifera. 



EPIDERMIS. (Em, epi, over or upon ; hepjxa, derma, skin.) The 

 fibrous, horny, external coating of shells, called by the French, 

 "Drop marin," or marine cloth. Lamarck objects to the name 

 Epidermis because he does not consider the substance as answer- 

 ing to the cuticle or scarf skin of the human body, but more 

 analogous to the nails and hair. Gray calls it the Periostracum, 

 from the membranous skin covering the bones of quadrupeds. 



EPIPHRAGM. The membranaceous or calcareous substance by 

 which some species of molluscs close the aperture of the shell, 

 when they retire within it to hibernate. When the animal 

 wishes to come forth from his hiding-place, again to breathe the 

 air, the edges of the Epiphragm are detached by a chemical pro- 

 cess, so that it drops off. The name Hibernaculum has also been 

 given to this covering. It must not be confounded with the 

 operculum, which is a permanent portion of the shell, and is 

 used as a door, fitted to the foot of the animal and moved at will 

 to pen or close the aperture of the shell, whereas the Epiphragm 

 is produced for the occasion from a mucous secretion of the ani- 

 mal and dissolved at the edges when no longer wanted, when it 

 drops off. 



EPISTYLA. Sw. A subgenus of the genus Helix. E. conical. 

 Sw. Helix Epistylium, fig. 281. 



EPONIDES. Montf. A genus of microscopic Foraminifera. 



EQUILATERAL. (^Iquus, equal ; latus, side.) Equal-sided. A term 

 applied to bivalve shells, when a line drawn down perpendicularly 

 from the apex would divide the shell into two equal parts. Ex. 

 Pectunculus pilosus, fig. 134. 



EQUIVALVE. (TEquus, equal ; valva, a valve.) A term applied 



