ZOOLOGY. 329 



Prof. Agassiz made the following observations on the rate of increase 

 and other characters of fresh-water shells, Unios. 



To determine their rate of growth, he had collected large numbers 

 during every month in the year ; he always found many series of 

 shells of different sizes, all of a size in each series, the whole suite of 

 specimens representing all the intermediate sizes, and, as he believed, 

 the rate of growth and annual increase. Though different species 

 breed at different seasons, none breed more than once a year, as is 

 proved by examination of the gills in which the eggs are deposited. 

 The small shells, less than an inch long, have generally been regarded 

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

 with en-o-s at this small size, and considered them as from seven to 



oo 



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. Mr. Lea separated the same as 

 Symphynota, 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 Mississippi, though described under 

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

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

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

 gradlis, 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 pounds, 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 four hundred species of fresh-water 

 shells in America, 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 striae correspond to a year's increase, if a species can- 

 not 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 local- 

 ities, and specimens attain the dimensions which Prof. Agassiz attrib- 

 uted 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 

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