12 



Also that the shells of a given dorso-ventral diameter are 

 longest at the Eddy stone and roundest at the Firth, of 

 Forth. Davenport concludes from the numbers that the 

 ancestral Pectejis 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. 

 maximus whose dorso-ventral diameter was 7"75 cm. and 

 antero-posterior diameter S'6 cm. had an indicated age of 

 three and a half years. 



The two valves are joined 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 ligament is also triangular, the apex 



