‘ 
d44 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 
Also that the shells of a given dorso-ventral diameter are 
longest at the Hddystone and roundest at the Firth of 
Forth. Davenport concludes from the numbers that the 
ancestral Pectens had a relatively greater dorso-ventral 
diameter, and that modern ones are becoming longer, 
since the measurements indicate that change. The 
variations recorded with regard to most qualities and the 
size of shells indicate that the Eddystone and Firth of 
Forth forms are the extremes in a regular series, the Irish 
Sea specimens being intermediate. The difference in . 
latitude means a difference in temperature, and probably 
also in the density of the water. 
By means of the ribs and their secondary thickenings 
on the inner surfaces of the shell, the two valves interlock 
and shut closely along the ventral margin. The external 
ribs and grooves are sculptured with well-marked striae, 
radiating from the umbo. ‘They are due to the presence 
of minute denticles arranged regularly in rows. ‘There 
is also a prominent concentric marking as if the shell was 
made up of a series of lamellae. These are much more 
pronounced in places forming definite rings which, since 
they occur very regularly and in the same positions, may 
be considered as indicating the age of the shell. <A P. 
maaimus whose dorso-ventral diameter was 7°75 cm. and 
antero-posterior diameter 86 cm. had an indicated age of 
three and a half years. | | 
The two valves are joimed along the hinge line by a 
narrow external ligament, present in addition to the 
thick internal ligament for the opening of the shell. The 
former simply unites the two valves and acts as hinge. 
The internal ligament is triangular in section, and in 
appearance like dark brown indiarubber. It fits into, 
and is attached to, the valves in deep triangular pits. In 
side view this heament is also triangular, the apex 
