330 ANNUAL OF SCIENTIFIC DISCOVERY. 



bridges, beyond which the folds of the gills are closely adherent to 

 each other, and at this season the outer gill is longer than the inner in 

 both sexes. Other characters are furnished by the gills, according as 

 they are attached to the foot or the transverse muscles. Prof. Agassiz 

 said that he had been led by these observations to a division of the 

 Naiades into natural genera, from the structure of the animal, as well 

 as shell, which had not been so well done hitherto from the shell alone. 

 He proposed to include under one genus, Unio alatus, U. fragilis, U. 

 gracilis, U. Ohioensis, U. Icptodon, U. Sayii, U. compressus, and U. 

 rectus. Of Unio alatus he remarked that specimens from the Western 

 waters, and from Lake Champlain, present differences in the teeth on 

 the hinge, and in the general configuration of the shell, but not, in his 

 opinion, enough to make a difference of species. U. gracilis and U. 

 fragilis, by some considered separate species, are only distinguished 

 by similar differences. To another genus, Prof. Agassiz proposed to 

 refer U.fiexus and gibbosus, from characters based on the more or less 

 circumscribed disposition of the essential internal organs. In conclu- 

 sion, Prof. Agassiz said that the fact that the Unios are perfect at the 

 time of birth, although very small and of extreme delicacy, led to inter- 

 esting speculations, how it could have been possible for so widely ex- 

 tended a family to have been distributed by any influence not primitive. 



WEIGHT OF SHELLS AND THEIR DISTRIBUTION AS REGARDS 



SIZE. 



IN his Contributions to Conchology, No. 4, Prof. C. B. Adams gives 

 a table showing the weight of shells of certain species of Colimacea. 

 Several shells were weighed at a time in accurate balances, and the 

 only error possible arises from the fact, that in the very small species 

 the animals are usually dried up in the shells. Those shells having 

 the smallest weights are Pupa Jamaicensis, .003 grain ; Achatina iota, 

 .0025; Helix apex, .0039; Stoastoma Blandianum, Bulimus minimus, 

 and Pupa milium, each of which weigh only .004 grain ; and Pupa 

 minutissima, only .00467. Truncatella fusca weighs .00667 grain ; 

 Stoastoma Redfieldianum, .0078. The other shells, of which forty-six 

 species are given, vary from this up to about 2 grains, though one 

 weighs as much as 71.41 grains; by far the larger portion weigh 

 considerably less than a grain each. 



Pupa minutissima is the least of the European land shells with 

 which we are acquainted, and P. milium is the least of the species 

 which inhabit the United States. Three of the species of the land 

 shells of Jamaica, Helix apex, Pupa Jamaicensis, and Achatina iota, 

 are, it will be seen, smaller than either of these, the latter being prob- 

 ably the least of ail the known species of terrestrial mollusks. It fol- 

 lows that the least of all known terrestrial shells exist in tropical re- 

 gions. An investigation of the marine shells collected in Jamaica 

 also shows a large number of extremely minute shells. From the fact 

 that the larger species occur only in tropical regions, it has been too 

 hastily inferred that only temperate regions contain a large portion 

 of very small species. The difference of zones is not that one ex- 



