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PROCEEDINGS OF THE NATIONAL MUSEUM. 



247 



The opinion is occasionally expressed in scientific literature that the 

 shell is a " mere secretion of the mantle." This usually proceeds from 

 some person who has not well studied the molluscau shell, or who is 

 of the age when one knows more than at any subsequent*period. 



Such a statement is one of those half-truths which are more danger- 

 ous than pure error, since the ballast of truth they contain will enable 

 the error to navigate some distance, while the uufreighted error would 

 capsize at once. 



The shell is in one sense the product of secretion from the mantle, as 

 the mammalian tooth is derived from the ectoderm of the jaw, or the 

 skeleton from the periosteum and cartilages. Both are that and much 

 more. It would be as reasonable to say that a steam-boiler, in process 

 of construction, is the product of the boy inside who holds the rivet- 

 heads, as to claim that the shell has no more significance than is im- 

 plied in the term " secretion of the mantle." 



The original theoretic protocouch may have been so, but as soon as 

 it came into being its development was governed by the physical forces 

 impinging upon it from all sides, and through it influencing the growth 

 and structure of the soft parts beneath. The Gastropod shell is the re- 

 sult of the action and reaction between the physical forces of the envi- 

 ronment and the evolutionary tendencies of the organic individual, In 

 the Pelecypod we have the mechanical stresses and reactions of one 

 valve upon the other added to the category of influences. To a consid- 

 rable extent it is doubtless as true that the animal is molded by its 

 shell as it is that the shell is shaped by the soft parts of the animal. 

 This results in that correlation of structure which has enabled stu- 

 dents to, in the main, correctly judge of the relations of mollusks by 

 their shell characters, when the latter were intelligently studied and 

 properly appreciated. 



Class PELECYPODA. 



I. Order Anomalodesmacea. 

 Suborders. 



1. Solenomyacea. 



2. Anatinacea. 



3. Myacea. 



4. Eusiphonacea. 



5. Adesmacea. 



II. Order Prioxodesmacea. 

 Suborders. 



1. Nuculacea. 



2. Arcacea. 



3. Trigoniacea. 



4. Naiadacea. 



5. Mytilacea. 

 C. Pectirtacea. 



7. Anomiacea. 



8. Ostracea. 



