54 
mkasurkmk:st ov isivalves. 
The Length of Bivalve shells, if the funnatioii and disposition of 
the ar.imal and its organs be accepted as guides, is the distance from 
the anterior to the posterior margins, not from the nmbo to the 
front margin as proposed by Dr. Jeffreys, the oral extremity of the 
animal being placed at the anterior end of the shell, and the 
alimentary canal terminating near the i)Osterior margin ; the 
Breadth, is the diameter transversely to this, that is from the 
dorsal to the ventral margin — the length as proposed by Dr. 
Jelfreys ; and the Thickness is considered to be at the greatest 
external convexity of the valves. 
IMuscular Impressions or Scars. 
The more or less irregnlarly shaiied and depressed mnscular scars, 
or impressions, especially noticeable on the internal snrhice of bivalve 
shells, indicate the places where the muscles of the body are affixed 
to the substance of the shell and the points where the animal and 
shell are orgaidcally connected together. 
In (tastropods, the columellar or retractor muscle, corresponding 
to the posterior retractor of the 
bivalves, leaves a double, or in some 
spirally coiled species, a single but 
not very conspicuous muscular scar 
upon the columella, about the dis- 
tance of a whorl from the aperture. 
In Ilell.r j)nm((tla it is a fan-shaped 
or triangular impression, extending 
spirally round the columella for half 
a volution, its further margin being 
indicated by a slig 
ridge, the acute angle of the cicatrix 
being directed basally, and its upper 
margin extending slightly upon the base of the preceding whorl. 
In the Pelecypods, the principal muscular scars are the anterior 
and posterior adductor impressions. In A iiodunta cj/rjiii'd the anterior 
adductor scar is placed at the anterior end of the shell, the posterior 
adductor scar at about the same distance from the posterior margin, 
between the posterior end of ligament and the posterior margin of 
htly perceptible 
Fig. 130 . — Helix pomatia L., 
Nieder K.nufungen, near Cnssel, 
Collected liy .Mr. P. W. Munn. 
Showing tlie character and position 
of the scar (s), indicating the point of 
attachment of the columella muscle (from 
a section cut by -Mr. F. Rhodes). 
