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V 



as of only a year's growth and as immature ; lie found them filled 

 with eggs at this small size, and considered them as from seven to 

 nine years old instead of one, and as mature. 



The Naiades have until recently been studied chiefly by amateurs, 

 and not by naturalists, and from the shells alone. Rafinesque made 

 a good beginning with the Kentucky species, separating Unio alatus 

 as the type of his genus Metaptera ; JNIr. Lea separated the same as 

 Sj/niphjnota, uniting under it, however, species entirely dissimilar. 

 In Metaptera (Raf ) the inner gill is united at the upper margin with 

 the side of the foot, there being no communication between the foot 

 and gill cavities, as occurs in U. complanatus^ so that the eggs must 

 pass back of the gill and by a very circuitous course ; the hind part 

 of the gill only is filled with eggs, in a kind of pouch, and the edge 

 of the mantle opposite is ciliated, evidently for the physiological pur- 

 pose of securing an ample supply of water, in itself a good generic 

 character. The species are the M. alata, the same from the Alabama 

 and the rivers flowing into the ISIississippi, though described under 

 various specific names in different localities, as U. Alahamensis and 

 Poulsonl; M. Ohioensis or U. Icevissimus of Lea, from rivers empty- 

 ing into the Ohio and upper Mississippi and IMissouri ; and M. 

 gracilis, also from the Northern States. In their early coming to 

 maturity this family is similar to fishes ; the pickerel of the Swiss 

 lakes, which attains a length of three or four feet, and a weight of 

 twenty to thirty lbs., spawns under a foot in length and a pound in 

 weight ; alligators also lay eggs when quite small. 



Dr. Gould remarked that this method of examining shells must be 

 very fruitful in results. At first the animals of shells were not studied 

 at all ; Mr. Lea finds now 400 species of this family in America alone, 

 whereas not many years ago only about twenty were known all over 

 the world. He stated that there was a great confounding of species 

 and even of genera among Unios. He inquired if the striaB corre- 

 spond to a year's increase, if a species cannot breed at the age of one 

 year, and what proof there was that these small shells were seven to 

 nine years old ? He had found shells which certainly grew to this 

 size in a single year in favorable localities, and specimens attain the 

 dimensions which Prof. Agassiz attributed to a life of thirty or forty 

 years, in three or four years. 



Prof. Agassiz replied that the finding of definite sizes at different 

 months without any intermediate degrees in each series had satisfied 

 him that the layers of increase were annual. Some species grow 

 rapidly for the first few years and then slowly, and others in a uni- 

 form manner ; they also grow more rapidly in some waters than in 

 others, so that the dimensions observed in one river are no guide for 

 those in another. All of the large shells found in waters which had 

 flowed for only three or four years might not have grown from eggs 



