GENUS CNIO. 261 liver, intestines, and an arterial and venous circulation, so complete as to excite our greatest admiration. How totally different is the rectus of Lamarck from the irroratus? ( nobis). The first is four times the width of its length, whilst the latter is longer than broad. The one is broad rayed, in fine specimens ; the other possesses dotted lines universally. The triangularis of Barnes is entirely dissimilar to the nasutus of Say, as is also the circulus, herein described,

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... m the lanceolatus (nobis) ; and the same may be said oi peruvianus and pictorum. Two species could * Swainson says, " Indeed so much uncertainty hangs on the shells of this genus, that the species can only be fixed by ample descriptions and very correct figures."-2rooL Illus. Vol. I. t. 57. * In a letter addressed to me by William Cooper, Esq., an intelligent naturalist of New York, he says, " There are now, I think, not less than thirty North American species of Unio well established, and perhaps seven or eight more. That they are species, each perpetuating its peculiar form, subject to certain variations, but permanent within fixed limits, seems tome the most rational opinion, although some of our most judicious naturalists think otherwise. Your account of the animal of the U.irroratus -nSovA^a strong argument in favour of this belief, for it proves that to be beyond doubt as distinct a species as any in any class of animals. Yet this may always be known with certainty by the shell, which, though so well characterised, is not, however, more different from the rest of the genus, than they are from each other, and frequently still less so. If, therefore, this difference is found to be constantly indicative of a species in one instance, it must also be in others. I believe that our lakes and rivers contained the same form of shells at the creation and ever since that they do at this day. If they are hermaphrodite per se, as is said of them, it could not be otherwise ; and if the contrary were admitted, natural history would not deserve the name of a science." VOL. III. 5 L * I reverse Lamarck's anterior and adopt Cuvier's as herelofore. t Purpurcus of Say. * I use this term for the elevated ridge which passes from the beaks to the posterior margin.