NAIAD.E. 



NAIADJE. 



1030 



mantle baa its margins, which cannot be reflexed, freely open almost 

 throughout, and the siphons are only indicated by a difference in the 

 structure of the posterior borders, the shells vary greatly in form and 

 aspect Those of our few native species are more or less oblong 

 and depressed ; but so great is the variation among foreign species, 

 that an American naturalist observes, there seems to be a representa- 

 tion of the forms of all the genera of marine bivalves in this tribe. 

 All the Unionida are equivalve. They are covered with an epidermis 

 which is often remarkable for brilliancy of colour, a feature also of 

 the pearly surfaces of the insides of their valves. 



The variation of the hinge is very great in this tribe, and its value, 

 as a source of character, has been a subject of much discussion among 

 conchologists. Some, as Mr. G. Sowerby, have gone so far as to pro 

 pose the union of all the genera in one, whilst others, as Mr. Swain- 

 son, have maintained the other extreme, and not only divided the 

 tribe into numerous genera, but have even grouped them into sub- 

 families. The tribe, as a group, is a very natural one ; the genera 

 are, we fear, too nearly inclined to artificial sections, and of the 

 extreme opinions that of Mr. Sowerby approaches nearest the truth. 

 There are however in the family a few types of character, round which 

 a number of species seem to group themselves naturally : the sections 

 Unto and Anadon, to which, with most authors, we refer our British 

 specie*, are of this nature, and may conveniently be retained. The 

 value of the generic divisions at best however in this tribe is not equal 

 to that awumed by such sections in most of the other tribes we have 

 passed in review. 



The L'nionidcr all inhabit fresh-waters ; but few comparatively are 

 found ia the Old World, by far the greater number being inhabitants 

 of America. In the United States, alone there are more than two 

 hundred species. The collections made by Major Cautley and Dr. 

 Falconer among the Siwalik fossils would seem to indicate that during 

 tertiary epochs the species of the Old World were more numerous 

 than they now are. The tribe ranges far back in time. All the species 

 are very variable, and it is extremely difficult to assign their precise 

 diagnosis. 



The bisexuality of the animals of this family was first, we believe, 

 observed by Professor Bell in Europe and by Dr. Kirtlaud in America. 

 The shells of the females are more ventricose than those of the males. 



The distribution of the British Fresh- Water Muscles is peculiar 

 and significant of the four species we posses. Three are members of 

 the genus Unio and one of Anodon. The last is generally distributed 

 through England, Scotland, and Ireland : one of the Uniones, that 

 representing the section Maryaritana, or Alasmodon, is partially dis- 

 tributed in the three countries, confining itself to the mountainous 

 portions and the rapid streams which flow from them. The remaining 

 two are confined to England, and abundant mainly in the south and 

 east. On the continent the Alatmodon abounds in the Scandinavian 

 rivers beyond the range of its fellows, which however have an almost 

 universal distribution through central and southern Europe. The 

 former species does not range south of the Alps. Taking the features 

 of this distribution into consideration, it seems as if the Unio maryariti- 

 fera had migrated southward from some ancient northern centre, whilst 

 the other Uniona and the Anodonta advanced westward and north- 

 ward, with unequal pace however, since only the last invaded Ireland. 

 The fossil contents of the fresh-water tertiaries contemporaneous with 

 or immediately preceding the period of the drift, when the greater 

 part of the British Isles was under sea, corroborate this view, for in 

 them we find associated together the Vnio tumidus and U. Pictorum 

 and the Anodonta cygnca. (Forbes and Hanley, 'History of British 

 Mollusca.') 



The North American rivers abound with this family; and Dr. Lea 

 of Philadelphia has contributed most largely to our knowledge of the 

 habits of these animals, and has made great additions to the species. 



Dr. Lea, in his ' Preliminary Remarks,' notices Lamarck's statement 

 that the animal of Anodon, which is essentially the same with that of 

 Ifnio, is hermaphrodite, and seems viviparous ; for the eggs pass into 

 the oviduct placed along the superior branchiae, where the young are 

 found with their shells complete. He then tells us that he dissected 

 a specimen of Anodonta undulata nearly three inches long, and found 

 the oviducts charged with about 600,000 (as nearly as he could calcu- 

 late) young shells perfectly formed, both valves being distinctly visible 

 with the microscope. In his second volume, bearing the title of 

 ' Observations on the Genus Unio,' &c., the same author informs us 

 that, believing that the oviducts would present the means of discrim- 

 ination in some species, he having found them to be so very different 

 in Unio irrot-atiu, his attention had been particularly addressed to 

 these organs in the few and small species of his Vicinity. Whilst 

 engaged in this investigation, Dr. Kirtland of Poland, Ohio, informed 

 Dr. Lea of his ability to distinguish the female and male shells of the 

 same species, without having recourse to the included animal ; and 

 xhortly afterwards Dr. Kirtland's paper on the subject appeared in 

 the ' American Journal of Science and Arts,' vol. xxvi. Dr. Lea's 

 attention now became more addressed to sexual characters, and he 

 states that a very short series of examination satisfied him fully as to 

 the establishment of the difference of sexes. The female, sustaining 

 her very large burthen, naturally requires, he observes, more space 

 within the valves ; hence an enlargement of the posterior portion of 

 the 8"-cll is generally found, differing in its form in various ppccit'S. 



The following figures, representing the oviducts of the species whoso 

 names are printed under the cuts are given by Dr. Lea. 



Vnio irroratus. 



1, soft parts, showing interior of oviduct ; 2, showing exterior of oviduct, tlie 

 mantle being removed ; a, mouth ; ft, great anterior muscle ; c, right superior 

 branchia ; d, great posterior muscle ; e, inferior right bronchia ; /, right oviduct ; 

 g, foot j 1, superior left branchia ; i, interior view of oviduct ; 3, 4, shell. 



Unio ochraceus. 



Vnio cariosity. 



Dr. Lea remarks that the mass of the lobes in Anodonta undulata, 

 differs from that otA.flumatilia, in presenting a darker appearance ami 

 a very curious arrangement of the oviducts. The ova are placed in a 

 kind of sac lying across the lobe, and presenting one end to the stomach 

 and the other to the mantle of the animal. They lie so close together 

 as to take the form on the exterior, like the cells of a honeycomb. 

 This, Dr. Lea says, is of course produced by pressure. Some of the-e 

 sacs, when carefully removed, were' found to contain as many as twelve 

 ova, each with a perfect living shell ia it, having a brownish epidermis 



