MOLLUSC A— PELECYPOD A. 363 



one for the entrance of the water and food particles, and the other 

 for the exit of the water and waste products. 



In the development of the shell a primitive nuclear portion or 

 prodissoconch is always present, though not always recognizable. 

 In the majority of forms it has the general characters of a nuculoid 

 pelecypod with a taxodont hinge structure or provinculum. This 

 is later replaced by the true dentition, of which the taxodont type 

 retains the primitive character of the provinculum. The inference 

 from this is, that the pelecypod, ancestral to the types with a 

 prodissoconch as described, must have had in general, characters 

 similar in its adult stage to those of the prodissoconch. 



In their early stages marine pelecypods like brachiopods are 

 free-swimming meroplanktonic larvae, which at certain seasons 

 swarm in the pelagic district, and are thus widely distributed. 

 Fresh and brackish waters also abound in pelecypods, but on the 

 whole the number of species is relatively small. 



In time, pelecypods are distributed from the Cambric, where 

 they are rare, to the present, where they appear to have reached 

 their acme of development, though many of their most bizarre 

 types were developed in the Comanchic and Cretacic. 



Three orders are generally recognized, Prionodesmacea, Anoma- 

 lodesmacea and Teleodesmacea ; under the aberrant division of 

 Palaeoconcha, which is probably not a natural group, are united 

 many primitive and perhaps degenerate types. 



Literature. 



Refer also to General References, p. 7. 



Descriptive Literature. — Selected Works. 



1847-1885. Hall, James. Pal. N. Y., vols. 1-5. (Vol. 5 is exceed- 

 ingly important for discussion of middle and upper Devonic pelecy- 

 pods. ) 



1864-1869. Gabb, W. M. Geol. Surv. California, Pal. 1 and 2. 



1873-1893. Hall, Whitfield, Meek, Ulrich. Pal. Ohio, 1, 2, GeoL 

 vol. 7. 



1876. Meek, F. B. U. S. Geol. Surv. Terr. 9 (Cretacic). Inval- 

 uable for its discussion of Cretacic genera and species. 



1884. White, C. A. and Heilprin, A. U. S. G. S. Ann. 4 (Fossil 

 Ostreidae of North America). 



1886 and 1907. Whitfield, R. P. and Weller, S. Pal. N. J., 1, 3, 

 4 (Paleozoic to Tertiary). Also Mon. U. S. Geol. Surv., IX. and 

 XXIV. 



