286 



SHELLS AND SHELL-FISH. 



PART I. 



The type which we consider unknown, is that which 

 leads immediately to Iridina, and would consequently 

 be deficient in some of the teeth. The other four are 

 such obvious representations of their prototypes among 

 the Unionin^, that we need not detain the reader by 

 any additional remarks. 



(267.) Of the iRiDiNiE, only three typical species have 

 been yet discovered ; and these, we believe, are all from 



the river Nile. (/. elongata, 

 fig. 60.) The form of the 

 whole group has been before 

 stated ; the shell is almost of 

 equal breadth throughout, 

 with the posterior end nearly, 

 if not quite, as broad as the 

 anterior. This great elongation of the shell must extend, 

 of course, to the animal; and for this reason we admit 

 into the genus Iridina a singular shell from the same 

 river {Iridina Nilotica Fer.), but which has the hinge 

 line only '^^ slightly crenulated at the umbones."* Here 

 it is that Nature, as it were, is hovering between the 

 confines of this type and the Anodontince, and plainly 

 intimates to us which will be her succeeding group. 



(268.) The ANODONTiNiB form a more numerous 

 sub-family than the two last. Tropical America ap- 

 pears richer in these shells, than any other part of the 

 world. Although one of the sub-genera is peculiar to 

 Asia, the typical form (represented by our common 

 Anodon anatinus) occurs both in Europe, Asia, and 

 North America, but under different modifications, which 

 will render it necessary to distinguish them as sub-ge- 

 nera. We arrange the whole under the five principal 

 genera of Lamproscapha Sw., Symphynota Lea, Anodon 

 Lam., Hemiodon Sw., and Patularia Sw. The first of 

 these contains such shells as the A. ensiformis, siliquo- 

 sum, &c.f , which have the shape of Iridina, but are 



• Zool. Journ. i. 55. The name of Pleodon cannot be substituted for that 

 of Iridina, as applied to the typical species. 



+ From Brazil, figured in Spix and Martius's Testacca Fhiviatilia, SjC^ 

 published in 1827. 



