149 
PERNA quadrata. 
TAB. CCCCXCII. 
Spec. Char. Quadrilateral,, one side shorter than 
the other three ; valves gibbose„ unequal, the 
shorter side very concave., bounded by two 
obtuse carinae ; beaks prominent. 
A remarkably square and convex Perna : the crena- 
tures in the hinge line, which fills one of the sides, are 
few and large. The shell, to judge from the remaining 
portion of the spar that filled the space it once occupied, 
was very thick, and so concave on the side beneath the 
beaks as to form there a heart-shaped cavity, bounded by 
ridges that curve gently along the beaks : in the centre 
of this cavity it is probable the byssus passed : the beak 
of the larger valve projects a good deal, the other is 
shorter ; they are both pointed, and nearly straight. 
This is little more than a cast in compact Limestone 
containing a little Mica ; but as the characters are very 
conspicuous, and as it is probable the outside will not 
easily be detached from the stone when other specimens 
are found, we have not thought it necessary to postpone 
the figure. 
It is from the Cornbrash at Rulwick, and was found 
by J ohn Hogg, Esq. 
