OCR Text |
Show 378 OR. H. L. JAMESON ON THE [Apr. I 6, described by Linnaeus as Mytilus margaritiferus ; and by reference to the descriptions in the tenth edition of the Systerna and in the Mus. Peg. Ulr., as well as to the type specimens in the Linnean Society's collection, I found that this surmise, made before Mr. Pace's'return 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, j| of the total anteroposterior 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 vars. 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-like ; 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 iu 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 prolongations, 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 exterior, and their nacre has a very white and silvery lustre, margin of nacre white or golden ; N e w Guinea shells are darker than Queens- |