274 SHELLS AND SHELL-FISH. PART I. 



and crassus of Barnes, and the undulata and Dombeyana 

 of Valenciennes, are varieties of Le Sueur's plicatus; 

 but as no proofs, so far as we know of, have been brought 

 forward for this opinion, we must withhold our belief 

 from it, more especially as the same author esteems the 

 Unio sinuata of Lamarck the same as the Mya marga- 

 ritifera of Linnaeus, and the Alasmodonta arcuata of 

 Barnes. These three shells are now before us ; and we 

 consider them not only of different species, but actually 

 of different genera : the first is a Unio, and one of the 

 types of Potomida; the two latter are distinct species 

 of Alasmodon. Furthermore, Lamarck has not given 

 the name of sinuata to the margaritifera of Linnaeus ; 

 for he expressly calls the latter shell, which, we believe, 

 is peculiar to England, Unio elongata. We are not fond 

 of adverting to errors ; but where they have been com- 

 mitted by writers who, on many points, deserve honour- 

 able mention, it is absolutely necessary to show the 

 fallacy of the principles of arrangement from which 

 they have originated. 



(257- ) On the remaining sub-genera of UNIO namely, 

 Cunicula, Ligumia, and Megadomus, our remarks must 

 be very brief. The first is at once distinguished by the 

 compression of the summits or bosses, and by being 

 always more or less angulated on the posterior slope : 

 planulata, cuneata, and rubiginosa are typical examples, 

 having the lower lateral tooth remarkably thick, the 

 singular foliacia of Say obviously representing the Unio 

 cornutus ; Lamarck's purpurata seems to lead imme- 

 diately to the sub-genus LIGUMIA, where the general 

 form of the shell is nearly as much elongated as any of 

 the IRIDIN.E. We consider Lamarck's recta (fig. 55.) 

 as the type, although we think this is also the place for 

 the gibbosa and other elongated species, whose outer car- 

 dinal tooth is not erect, and both teeth more in unison 

 with those of Cunicula and Unio. In all these the shape 

 and direction of the 'cardinal teeth are quite different from 



* Lea on the Genus Unio, Am. Trans vol. iii. N. S. 1829. 



