10 
CLASSIFICATION. 
gilLs. The class may be first divided into three Orders, according 
to the nnmher and development of the Adductor muscles. This 
mode of classification, though perliaps not perfectly satisfactory, 
seems preferable to the various other methods of division that have 
U.T 
Fig. 11. — An Isoniyate, Integripalli.'ite Pelecypod. 
Anodonta cygmea (L.), left valve, 
Clumber Lake, Notts., collected by Mr. C. T. Musson, F.L.S. 
a. ad. anterior adductor scar; p.ad. posterior adductor scar ; a.r. anterior pedal retractor scar ; 
p.r. posterior pedal retractor scar ; u.r. umbonal or lesser retractor orretentor scars ; a.p. anterior 
pedal protractor scar, the abdominal retentor scar of Clessin ; /./. palbal line. 
at different times bee 
a ad 
Fig. 12. — A Heteromyate 
Pelecypod. 
D. polyniorpha (Pallas), 
Gloucester Canal, Stroud, 
Coll, by Mr. E. J. Elliott. 
a. ad. ant. adductor scar ; 
p.ad. post, adductor scar ; 
post, pedal and byssal 
retractor scar ; /./. pallial 
line ; 1. ligamental pit. 
n proposed : the Monomya, or one-muscled as 
the Oyster ; Isomya, in which those muscles 
are two in number, and appro.ximately e(iual 
in size, as e.xemplified in the Unioitidw] and 
IIeteromya, in which, although two muscles 
are still developed, the anterior one is much 
smaller than the postei’ior one, as in Dreis- 
sensiu. Sub-orders are formed in the Isomya, 
by the presence or absence of a conspicuous 
sinus or indentation in the pallial line, in- 
dicating when present the possession of 
extremely long retractile siphons, and thence 
called SiNUPALLiA, as expressing the indi- 
cation the pallial line affords. The Inte- 
GRiPALLiA, to which all our species belong, 
have a non-sinuated pallial attachment. 
The classification adopted is given also in tabular form, so that 
the inter-relationship of the different groups can be understood at a 
glance. 
