OF BIVALVE SHELLS 



829 



XI] 



incomplete: the old carapace is retained, and the new, growing up 

 underneath it, adheres to it like a lining, and projects beyond its 

 edge: so that in course of time the margins of successive old 

 carapaces appear as "lines of growth" upon the surface of the shell. 

 In this mode of formation, then (but not in the usual one), we obtain 

 a structure which "is partly old and partly new," and whose suc- 

 cessive increments are all similar, similarly situated, and enlarged 



Fig. 403. Hemicardium inver- 

 sum Lam, From Chenu. 



Fig. 405. Section of Productus 

 '{Strophonema) sp. From 

 Woods. 



Fig. 404. Caprinella adversa. 

 After Woodward. 



in a continued progression. We have, in short, all the conditions 

 appropriate and necessary for the development of a logarithmic 

 spiral; and this logarithmic spiral (though it is one of small angle) 

 gives its own character to the structure, and causes the little carapace 

 to partake of the characteristic conformation of the molluscan shell. 

 Among the bivalves the spiral angle (a) is very small in the 

 flattened shells, such as Orthis, Lingula or Anomia. It is larger, 

 as a rule, in the LameUibranchs than in the Brachiopods, but in 

 the latter it is of considerable magnitude among the Pentameri. 



