270 SHELLS AND SHELL-FISH. PART I. 



and probably of ALASMODON, in the species mentioned 

 above. We say probably, because,, as there is no es- 

 pecial evidence to prove the fact, we only infer it from 

 this shell having a structure intermediate between no- 

 dosa and JEsopus ; hence, it occupies the same station 

 in its own circle, as Alasmodon does in that of the 

 whole family. 



(254.) So astonishingly variable, however, are the 

 shells of this family, that we may even carry our ana- 

 lysis still further ; and by following the series of spe- 

 cies which arrange with Unio Mytiloldes, we shall detect 

 the same analogies as are apparent in the last inves- 

 tigation. Thus, some of the species or varieties of 

 Mytiloides (as gibbosus} are almost equilateral ; others 

 pass into Ellipsis and sulcatus of Lea, thus assuming the 

 long shape of truncata arid IRIDINA ; next follows 

 cornutus, as a prototype of nodosa and Theliderma ; 

 and finally) Msopus brings us again to such shells as 

 U. obliqua, and the inequilateral varieties of Lea's gib- 

 bosus. Beyond this point, it is utterly impossible to 

 push analysis, except it be carried into the principle 

 by which the varieties of each species are regulated; and 

 this, we firmly believe, is in conformity with the very 

 same laws. We have seen, in fact, four varieties of 

 the Mytilo'ides, which assume four out of these primary 

 forms. Some are inequilateral (fig* 53.), some trigonal, 

 others are oval, and another oblong : whether there is 

 any one with small nodules to represent cornutus and 



_, its prototypes, we know 



^ ^ not, but we have no doubt 



that such a variety will 

 be found.* Mytiloides, 

 therefore, we consider 

 as the most typical 

 species of the whole fa- 

 mily, concentrating, in 

 its own variations, pro- 



* Since writing the above, we have seen a variety of Mytiloides perfectly 

 answering our theoretical description, in the hands of Mr. Stuchbury. 



