262 THE TRANSFORMATIONS OF THE ANIMAL WORLD 



in the course of their individual evolution. Jack- 

 son, who, on his side, has followed up the different 

 stages of development of a great number of forms, 

 has shown that the primitive shell commences as 

 a thin cuticle in the shape of an uneven saddle, like 

 the primitive shell of the Gastropods ; then a 

 deposit of lime occurs at the two extremities of this 

 membrane, forming a bivalvular shell, or primitive 

 prodissoconch. In all the genera in which it has 

 been observed, oysters, cockles, scallops, etc., 

 the prodissoconch is equivalvular, with a straight 

 hinge, void of teeth, with a rounded and not very 

 prominent top. In all cases the animal is provided 

 with two adductor muscles, and only becomes 

 monomyous* by subsequent modifications, which 

 likewise influence the direction of the hooks, the 

 position of the ligament, the fixation of the shell, 

 its ornamentation, etc. The prodissoconch often 

 remains visible and sharply distinct from the rest 

 of the shell up to a certain age. The development 

 of the teeth of the hinge is particularly interesting 

 to follow in certain families. In oysters, without 

 teeth at the adult stage, Munier-Chalmas was able 

 to observe on embryonic shells teeth ranged in 

 series, as in the existing Nuculas and Areas. This 

 type of hinge, with crenellated teeth, or Taxodont 

 type, appears to be, in fact, according to Jackson, 

 the most primitive form of the hinges of the Lamelli- 

 branchs ; the Nuculas, the Areas, the living Pec- 

 tunculus would thus be, in a way, the persistent 

 embryonic types of this order of Molluscs. On the 

 other hand, in the Pectinidee, also without cardinal 



* Single-muscled. ED. 



