378 DB. H. li. JAMESON ON THE [^Pl"- 1 6, 



described by Linnaeus as Mytilus margaritiferns ; and by reference 

 to the descriptions in the tenth edition of the Sysiema and in the 

 Mus. Reg. Ulr., as well as to the type specimens in the Linnean 

 Society's collection, I found that this surmise, made before Mr. 

 Pace'sreturn to England, was correct. I suggest the name maxima 

 for this species, as it reaches larger dimensions than the other 

 members of the genus. 



The shell is flatter than in M. margaritifera and of a different 

 shape ; the hinge being relatively much longer, ^ of the total antero- 

 posterior measurement of the nacre as a rule. The posterior 

 margin of the nacre makes with the hinge an acute angle (in old 

 shells a right angle) ; and a perpendicular to the hinge from its 

 posterior end would pass outside the nacre altogether, or coincide 

 for half the depth of the shell with its posterior margin. In 

 typical examples of M. margaritifera (but not in \wb. persica and 

 mazatlanica) such a perpendicular would cut off a small area of 

 the nacre posteriorly. 



The anterior margin of the nacre, ventral to the byssal cleft, 

 does not project so far forward as in M. margaritifera. 



The byssal cleft in the right valve is less pronounced than in 

 the preceding species, and becomes almost obliterated with age, 

 as the byssus is lost when the shell reaches a diameter of 15- 

 20 cm, 



The young shell is flat, thin, and plate-hke ; with age it becomes 

 massive, and may measure nearly afoot in diameter, and weigh 10 

 or more lbs. per pair. 



The lappet-like processes of the lip differ in form from those of 

 M. margaritifera, being broad at the base and gradually narrowing 

 to the rounded apex, while in the latter species they are more 

 oblong and parallel-sided. They are thinner and more fragile 

 and transparent in M. maxima than in M. margaritifera. 



Colour pale yellowish brown to deep ochre, with traces of radial 

 markings of dark brown, green, or red in the umbonal area. The 

 marginal region of the shell, together with the lappet-like pro- 

 longations, is marked by a series of characteristic dark and light 

 circumferential lines, about 1-3 mm. apart, which I have never 

 seen in M. margaritifera. The white spots of the preceding 

 species are quite absent ; when there are traces of radial markings 

 they are darker than the ground-colour. The inner surface of the 

 lip IS light golden brown or amber-coloured and almost transparent. 

 The nacre is silvery white, lustrous, with or without a golden 

 margin. 



As the geographical range of this species is much more restricted 

 than that of the preceding, it does not so readily split up into 

 well-marked geographical varieties, although at least seven local 

 types are recognized in the Trade. " Port Darwin," " West 

 Australian," and " Queensland " shells have a light yellowish ex- 

 terior, and their nacre has a very white and silvery lustre, margin of 

 nacre white or golden ; jSTew Gruinea shells are darker than Queens- 



