Chap. IX.] 



STRUCTURE OF SHELLS. 



37 







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 mam- 

 ruillated ; it is made up of ichorls (,-), separated 

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

 axis or coluiuella (i), an outer Up Co), an aperture (.), 

 and an anterior (c) and posterior (pc) caiial. (After 

 "Woodward.) 



