12 ! 
PECTEN. 
Gen. Char. A regular, eared, inequivalved bivalve, 
with contiguous beaks. The hinge toothless ; 
pit trigonal, receiving the internal ligament. 
One muscular impression. 
PECTEN quadricostata* 
TAB* LVI. — Figs, 1 and 2. 
Spec. Char. Triangular, nearly even, front semi- 
circular, margin notched. Convex valve ribbed, 
larger costa six*, three smaller between each. 
Posterior auricle large. 
The length is somewhat greater than the width; surface 
nearly smooth. There are regularly three small costa 
between each of the larger, making five sets of four ribs 
each : near the sides the costae are less regular and smaller. 
This is by favour of T. J. L. Baker, Esq. from Halldown 
near Exeter, out of the green sand. No. 2 is a fragment, 
perhaps of the same species, from the green sand at Chute 
Farm, in the parish of Horningsham, near Longleat, Wilts. 
They are both siliceous, the latter is much worn, as if rolled. 
This is perhaps the same as the fine large shell from near 
Stourhead, represented in British Mineralogy, tab. 183, 
in which the shell itself is Carbonate of Lime, not siliceous. 
The Rev. Thomas Rackett has found the same species at 
Aynswell hill in Dorsetshire, in which the shell is siliceous. 
* By some unaccountable accident, five only happen to be represented, 
which somewhat disguises the figure, but may be understood as now mentioned, 
