474 TERMINOLOGY OF 



from each other. The latter are sometimes buried in the 

 mantle and hidden. 



The anterior (a) and posterior (b) valves are semicircular 

 and nearly equal ; the intervening are subequal, oblong, and 

 transverse with sloping sides. " The posterior valve, which 

 is placed over the more important organs, is generally the 

 most fully developed, and is the homologue of the shell of 

 the Patella ; while the others, which are arranged in front of 

 it, are more imperfect ; and the front one is the most rudi- 

 mentary of the series." J. E. Gray. 



The external surface of each 

 Fig. 92. valve is divided into a centre 



and a right and left side. The 

 apex (Fig. 92 a), more or less 

 marked, occupies the centre, 

 and looks towards the poste- 

 rior extremity. Each side is 

 divided into two areas by a 

 line passing diagonally from the posterior aspect of the apex 

 to the anterior and lateral margin. They are easily distin- 

 guishable by being striated or granulated in opposite direc- 

 tions, the anterior area transversely, and the posterior area 

 longitudinally. (Fig. 92.) 



The inferior margin of the valves, imbedded in the mantle, 

 is marked with neat small notches, variable in numbers. 

 Their number may be counted by the porous lines which are 

 to be observed on the inner surface of the valves diverging 

 from the apex to the margin. 



The lateral or transverse margin is deeply sinuated in the 

 middle, and furnished with a thin prominent rounded pro- 

 cess on each side. The central sinus (Fig. 92, s) is minutely 

 serrulated, and the lateral lobes or wings (Fig. 92, I, I) are 

 also frequently so charactered. These lobes can generally 

 be seen from the under side without separating the valves. 

 There are none on the anterior valve. 



In the living animal these lobes are inserted into the sub- 

 stance of the mantle, and hence the valves are subject to its 

 contractions. By this means the snail can roll its shell into 

 the form of a ball more or less perfectly, like the wood-lice. 



The marginal band varies in breadth and sculpture exceed- 

 ingly. It is smooth or furfuraceous, shagreen ed, granular 

 or scaly, hirsute or spinous. The granules and scales are 

 arranged usually in quincunx. The spines are in most 

 collected into tufts.* 



* Gray in Ann. and Mag. N. Hist. xx. 69 ; and in Phil. Trans. 1847, 

 p. 141-5. 



