EXPLANATION OF TERMS. 221 



a prefex, signifying division. 

 Decussated. Crossed like an X ; from decussis, 



the mark X (ten). 

 Disk. The convex part of the valves between 



the urn bones and the margin, applied also 



to any extended or rounded surface ; as in 



Haliotis, from the Latin discus, a dish 



or platter. 

 Effuse. Having the lips separated by a sinus 



or gutter, so that if the shell were filled 



with water, it would flow out at the sinus 



before it reached the margin ; from fus us, 



poured out. 

 Epidermis. The membranaceous covering of 



some species of shells ; from the Greek 



em (epi) upon, and $ep/xa (derma) a skin. 

 Eroded. Gnawed out, from the Latin e, out, 



and rod ere, to gnaw. 

 Entire. The mouth is said to be entire, when 



the lips are not separated by a canal. 

 Equivalve. Having equal valves ; from equ us 



equal, and valve. 

 Equilateral. Having both sides equal ; from 



equ us, equal, and latera, sides. 

 Emarginated. Notched, or having the margin 



excavated by a sinus. 

 Ferruginous. The colour of rust or iron-mould ; 



from ferrugo, iron rust. 



Flexuous. Winding full of turns and meanders. 

 Fusiform. Shaped like a spindle; swollen in 



the middle, and tapering to each end; 



from fusis, a spindle. 



