136 
MACTRA dubia. 
TAB. CLX.---Figs. 2, 3, and 4. 
Spec. Cuar. Ovato-triangular, transversely elon- 
gated, smooth, thickened towards the margin, 
sides equal. 
Tours 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, Esy. 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 ovyalis. 
TAB. CLX.---Fig: 5. 
Spec. Cuar. Oval, equilateral, smooth ; thickness 
uniform. 
Toenarn 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, Ksqrs. 
MACTRA cuneata. 
TAB. CLX.---Fig. 7. 
Sprc. Cuar. Ovate, smooth, depressed towards the 
front, anterior margin acutangular. 
Nidee but little more than half the width ; the anterior 
side is largest and angular. 
These are from Bramerton-hill near Norwich, found 
by my late friend Charles Wilkinson, Esq. 
¥ Nothiug shews the necessity of nicety in description more than the 
difficulty of discriminating the recent from the diluvian or antediluvian 
species: the accordance in tine outside when worn, in most of these species 
and of Pennant’s figures, has conveyed an idea of all being the same. 
