CLASSIFICATION. 11 
account of the animal of the 7. caput serpentis to the ‘ British 
Mollusca,’ and for other general observations to Professor 
Owen’s paper on this family in the first volume of the ‘ Zoo- 
logical Transactions.’ 
Second Division. 
ACEPHALA LAMELLIBRANCHIATA. 
The animals of this group are strict hermaphrodites, though 
it is said that in some of them the sexes are distinct. We 
dissent from this view, and have assigned in another place our 
reasons for not concurring in this opimion. 
The anatomy of the internal organs of the entire tribe as 
to generalities is so similar, that it scarcely affords sufficiently 
decided generic distributive pomts; my anatomies of Pholas 
dactylus and Teredo megotara confirm this position; I have 
therefore had recourse to an arrangement which combines 
both internal and external organs, to assist the distribution of 
this numerous class into convenient groups. A divisional 
order has been attempted on ligamental bases of internal or 
external position ; but it has been found so unstable and arbi- 
trary, that if strictly followed, the most incongruous species 
would be associated: for instance, Mactra solida would march 
with Anatina pretenuis, and the Cardia with the Savicave. 
The disposition of the adductor muscles has been tried, and 
appears to be delusive and unsatisfactory, as most, if not all, 
Lamarck’s Monomye have two adductor muscles, though the 
volume of one is much greater than the other. We think the 
only true Monomye are the Pholadide, as we have shown in 
the memoir on the anatomy, and these are Dimye with that 
eminent zoologist. 
The teeth and foot as general guides are so variable as not 
to be available; the best of these aids is perhaps the greater 
or less closure of the mantle; this last we have adopted. 
It appears then that the animals cannot be allocated in a 
continuous natural order with perfect satisfaction by any of 
these modes : all that can be done by those who make use of 
such assistance, is to throw the tribes, genera, and species 
