EXPLANATION OF TERMS. 221 



in a shell ; from sepes, a hedge, and dis, 



a prefix, signifying division. 

 Decussated. Crossed like an X ; from decussis, 



the mark X (ten). 

 Disk. The convex part of the valves between the 



umbones 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 



7n (epi) upon, and $ep/>ta (derma) a skin. 

 Eroded. Gnawed out, from the Latin e y 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 t equal, and latera, sides. 

 Emarainated. 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 



