EMBRYOLOGY AND CLASSIFICATION. 311 



ing these specimens, and making constant eifoi'ts 

 to ascertain by what Mollusks they are laid. 

 The fact tliat among Acephala the bivalve shells, 

 which have two transverse bundles of muscles in 

 their adult condition, have only one in earlier 

 life, furnishes a satisfactory evidence of the lower 

 standing of the Monomyaria when compared to 

 the Dimyaria ; and a closer investigation leaves 

 no doubt that the one large transverse muscle 

 of the Oyster corresponds to the posterior muscle 

 of the Clam (Mactra or Mya). Again, the prev- 

 alence of the vertical diameter over the longi- 

 tudinal and the transverse in the young, while 

 later the longitudinal diameter takes the lead, 

 affords valuable information respecting the rela- 

 tive standing of short-bodied or rounded Bivalves 

 in comparison with their more elongated allies. 

 And this is in accordance with the inferior posi- 

 tion of the Brachiopods and Bi'yozoa. But the 

 classification of Mollusks is too little advanced 

 as regards the relative standing of their numer- 

 ous families to allow a more direct comparison 

 at present. 



In the type of Articulates the difficulties are 

 of another kind. The extraordinary number of 

 representatives in the classes of this type renders 

 a comprehensive view of their respective stand- 

 ing particularly difficult ; and yet, if we leave out 

 of sight the minor divisions, and keep in mind 



