262 SHELLS AND SHELL-FISH. PART I. 



ruption so great as to cast a doubt on the truth of this 

 proposition, or to admit the introduction of any shell- 

 fish that have been arranged in other families. On this 

 ground, and not for its typical perfection, we commence 

 our survey with Iridina, where the transverse or tubuli- 

 form structure of shell is so conspicuous : here the 

 cardinal teeth are wanting, being absorbed in one single 

 lateral tooth which occupies the whole length of the 

 hinge margin ; this lateral tooth being tuberculated or 

 crenated. Now, the only other examples where the car- 

 dinal teeth put on the aspect of lateral teeth, and are 

 also crenated, is in such shells as Hyria avicularis Lam., 

 where, in fact, the cardinal teeth * are so far removed 

 towards the side as to assume the aspect of lateral teeth ; 

 the interval between those on the other side of the shell 

 being occupied by a few irregular crenations, similar to 

 those, in short, of Iridina : on coming to Hyria corru- 

 gata Lam., these crenations disappear, the cardinal teeth 

 assume their true position immediately under the bosses, 

 and the passage thus becomes opened to Lamarck's Cas- 

 talia. The Indian Unio corrugata Lam. then succeeds, 

 and from standing at the confines of Hyria, connects 

 that group, in conjunction with other species more elon- 

 gated, with Mysca ovata of Turton. This brings us at 

 once to the genus Unio of Lamarck, where the chain of 

 aflUnity passes on from Mysca nasuta Sw. to the Unio 

 ossidens. The pre-eminent types of the whole then 

 succeed, as shown in U. cuneatus and the Mytiloides of 

 Raf. By means of U. JEsopus, we pass to Lamarck's 

 sinuosa, Unio gioas Sw., and so enter Say's Alasmodon 

 by the well-known Unio margaritifera of Europe. 

 From this point even the cardinal teeth begin to dis- 

 appear ; for A. undulatus and calceolus bring us close to, 

 and rugoftus within, the confines of Anodon : all that 

 now remains of the teeth is a slight elevation beneath 

 the hinge margin, as in Dipsus Leach ; and even this 

 finally disappears, as in the typical anodons. Gradu- 



* True cardinal teeth are placed immediately beneath the bosses, and 

 not on one side, as in Hyria avicularis. 



