136 
MACTRA dubia. 
TAB. CL X.—Figs. 2, 3, and 4. 
Spec. Char. Ovato-triangular, transversely elon- 
gated, smooth, thickened towards the margin, 
sides equal. 
This is between M. solida and stultorum ; it differs from 
the last only in being wider and thickened towards the 
edge : the lines of growth are conspicuous, and more par- 
ticularly so when the surface has been corroded, a cir- 
cumstance generally observable in bivalve Crag shells.* 
I received a specimen of this species nearly as large as 
the last, by favour of Dawson Turner, Esq. but broken ; 
I therefore figure smaller but more perfect specimens 
received since from the Rev. G. R. Leathes. I have had 
the same, smaller still, from Mrs. Cobbold at Ipswich, 
and from Woodbridge. 
MACTRA oralis. 
TAB. CLX.—'Fig. 5. 
Spec. Char. Oval, equilateral, smooth ; thickness 
uniform. 
Length about three-fourths of the width : its regular 
oval form distinguishes it : it is also rather deeper than 
M. stultorum. 
A Crag shell, sent me from Suffolk by Dawson Turner 
and W. J. Hooker, Esqrs. 
MACTRA cuneala. 
TAB. C LX — Fig. 7 . 
Spec. Char. Ovate, smooth, depressed towards the 
front, anterior margin acutangular. 
Length but little more than half the width ; the anterior 
side is largest and angular. 
These are from Rramerton-hill near Norwich, found 
by my late friend Charles Wilkinson, Esq. 
* Nothing shews the necessity of nicety in description more than the 
difficulty of discriminating the recent from the diluvian or antediluvian 
species : the accordance in the outside when worn, in most of these species 
and of Pennant’s figures, has conveyed an idea of all being the same. 
