624 R. J. HARVEY GIBSON ON THE 



a granular crushed layer, very irregular, and resting on a dense layer of con- 

 nective tissue. The epithelium, at least in sections, most commonly falls off, 

 and leaves the subjacent connective tissue exposed. 



Over the head and neck the columnar epithelium is composed of large cells 

 which have distinct nuclei. The subepithelial connective tissue layer is very 

 evident, and fibres are seen passing through it to the epithelial cells 

 (PL CXLIX. fig. 9). On surface view the cells present a granular mosaic. 



Protective System. — Under the head of protective system may be classed the 

 mantle, whose function in this relation is to afford protection to the functional 

 gills and the shell. The mantle has already been described under the respiratory 

 system. 



The shell, as has been already stated, is dome-shaped, and has ventrally an 

 oval outline (PL CL. fig. 22), the narrow end being anterior. The apex of 

 the dome is blunted, and the outline of the sides is curved. The apex of the 

 dome is nearer the anterior end (PL CL. fig. 23). 



Externally two series of lines are visible on the shell — (a) a radiating 

 series from the dorsal apex to the edge ; (b) a series concentric with the 

 ventral edge. The lines are of various degrees of coarseness, and some are 

 nodulated and tuberculate. 



Internally, the radiating lines are represented by bands of blue and yellow 

 of variable shade and width. These bands are crossed at intervals by con- 

 centric bands of dark colour more or less distinct in different individuals. 



The rim of the shell is chisel-shaped, the bevelled side being inwards. The 

 shell rapidly thickens from the edge, and then maintains a tolerably constant 

 thickness throughout. The edge is sharp and notched. 



The pallial line is visible as a pale belt of variable width (usually £ inch) 

 running in a sinuous manner along the shell about \ inch from its edge 

 (PL CL. fig. 22). 



Inside the mantle line at a short distance is the impression of the attach- 

 ment of the circular muscle connecting the foot and the shell. The impression 

 is divided, as is the muscle itself, into more or less distinct areas. The breadth 

 of the belt is tolerably constant (usually about \ inch.) The most anterior 

 muscle impression, one each side, is larger, and rounded anteriorly. The inner 

 border of the belt is more irregular than the outer. 



Within the impression of the circular muscle there is a belt of irregular 

 breadth and outline, generally broadest posteriorly. The belt marks the 

 attachment of the integument of the visceral dome. The impression expands 

 in front, and fills up the space left vacant by the absence of the circular muscle 

 impression. That space is about one-sixth the entire circumference of the shell 

 at that level. The remainder of the concavity of the shell is not touched by 

 the derm of the visceral dome. It is usually white, and lacks the lustre of the 



