Chap. IX.] 



STRUCTURE OF SHELLS. 



307 



majority of cases the exact systematic position of a 

 mollusc may be determined by the shell alone, so 

 marked are the 

 differences, and 

 so deep - seated 

 the essential cha- 

 racteristics. Geo- 

 logists believe 

 that there is no 

 evidence more 

 worthy of con- 

 fidence than that 

 which is afforded 

 them by the 

 shells of any 

 given deposit. 



The shell, 

 which owes its 

 growth to the 

 activity of the 

 outer cell-layers 

 of the mantle, 

 commences as a 

 pit or invagina- 

 tion of the outer 

 layer on the abo- 

 ral surface of the 

 larva ; this pit is 

 the shell gland 

 of Lankester, and 

 it secretes a vis- 

 cid body which 

 hardens on con- 

 tact with water ; this hardened substance is the earliest 

 rudiment of the shell, and even in the bivalved forms 

 it is at first a single saddle-shaped plate, which only 

 later becomes divided into two bilateral halves. 



Fig. 125. Shell of Triton, to explain the terms 

 used in the descriptions of Shells. 



The -shell is fusiform, in shape : its apex (A) is niam- 

 niillated ; it is made up of whorls (w), separated 

 by sutures (su) ; bw, body whorls There is an internal 



millated ; it is made up of whorls (w), separated 

 by sutures (su) ; bw, body whorls There is an internal 

 axis or'columella (i), an outer Up (o), an aperture (a), 

 and an anterior (etc) and posterior (pc) canal. (After 

 Woodward.) 



