EMBRYOLOGY AND CLASSIFICATION. dll 
ing these specimens, and making constant efforts 
to ascertain by what Mollusks they are laid. 
The fact that 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 Bryozoa. 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 
