IRIDINA. 167 



matter which entered the shell in a soft state, and has subse- 

 quently hardened, when, the shell dropping off, the hardened 

 substance which filled it is left to represent its internal form. 



INTERNAL LIGAMENT. A term used by some conchological 

 writers signifying that the ligament of a bivalve shell is placed 

 within the closed part of the hinge, so as not to be seen when 

 the valves are shut. But the substance, formerly called the in- 

 ternal ligament, is now distinguished from the true ligament 

 both in structure and use ; and is now more properly called the 

 cartilage, so that when the ligament is said to be internal, it 

 must be understood that the internal cartilage is unaccompanied 

 by any ligament properly so called, and when a shell is described 

 as having two ligaments, as in the case of Amphidesma, it means 

 that the two substances are so far removed from each other in the 

 hinge that they are no longer confounded together. 



INTERNAL SHELL is one which is enclosed in the soft parts of 

 the animal, as a bone is enclosed in the flesh of a human body. 

 The Limax, or common garden slug, which has a testaceous 

 shield beneath its mantle, is an instance of this. 



10. Lea. A genus composed of several species of fresh-water 

 shells which are considered as differing from Melaniae in having 

 the anterior termination of the aperture produced into a point in 

 some degree resembling the caudal canals of shells belonging to 

 the family of Canalifera, which are marine. Io fusiformis and 

 spinosus are described and figured in Lea's work on the genus 

 Unio. 



JODAMIA. Defr. A genus resembling Birostrites, except that in 

 Jodamia one valve overwraps the other, while in Birostrites the 

 circumference of the valves is equal. 



IPHIGENIA. Gray. A sub-genus of Clausilia, C. biplicata, &c. 

 Auct. Gray's Turton, p. 214. 



IRIDEA. Sw. A genus of " Hyrianoe," Sw. thus described : — 

 " Oblong ovate ; bosses small, depressed, sulcated ; inner car- 

 dinal tooth placed beneath the outer. I. granosa, Lam. En. 

 Meth. 248. fig. 9." 



IRIDINA. Auct. A genus belonging to the Nayadcs, and re- 



