1901,] MOTHER-OF-l^EARL OtSTEBSi 3?3 



another by hard-and-fast lines, owing to the absence of well- 

 marked diagnostic characters, and to the extraordinary amount 

 of geographical and casual variation. 



Probably the anatomical characters will, when known, be found 

 to be the best guides to the limits and affinities of some of the 

 species. 



It is possible to divide the known species and forms nito two 

 groups — the first of which has no traces of hiuge-teeth, the second 

 possessing a pair of minute tubercles anterior to the hgament, 

 and a small lamina in each valve representing a lateral tooth. The 

 latter group may be split up into three sections, according to the 

 form of the shell. 



' Division 1. Hinge without teetJi. 



Species 1. Maegaeitifeea maegaritieera. 

 Type in Mus. Linn. Soc. Lond. 



Mytilus margaritiferus, Linnaeus, 1760. 



Margarita sinensis, Leach, 1814, vol. i. p. 180, pi. 48 ; China. 



Margaritiphora mazatlanica, Hanley, 1855, p. 388, pi. 24. fig. 40; 

 Mazatlan, California (var. dist.). (Not Avicula fimbriata Dunker, 

 1852 ; for M. mazatlanica Hanley, Carpenter, 1857, p. 550 ; 

 Mazatlan.) 



Avicula cumingii, Eeeve, 1857, sp. 6 ; Lord Hood Island (var. 



dist.). ^ . 



Avicula barbata, Eeeve, 1857, sp. 9 ; Panama = var. mazatlayiica 



(Hanley). 



This species is distinguished from M. maccima, the only other 

 known member of this division, by its greater convexity and the 

 shortness of its hinge, and by its colour and markings. 



In typical examples the hinge-margin measures but little more 

 than half the length of the nacreous surface of the valve, from 

 the anterior to the posterior margin. 



The " rostrum " or anterior angle of the hinge, dorsal to the 

 byssal notch, is more distinctly marked off from the shell than in 

 M. maxima, and the posterior end of the hiuge usually meets the 

 posterior margin so as to form an obtuse angle (except m var. 

 persica and var. mazatlanica). There is no sinus in the posterior 



margin. • j. r n 



The anterior margin, ventral to the byssal notch, projects turtner 

 forward than in M. maxima, so that a line perpendicular to the 

 hinge at its anterior end Mould cut off a considerable area of the 

 nacre anteriorly (i- total antero-post. measurement). 



The lappet-like" processes of the lip are more numerous, more 

 crowded together, narrower, and relatively longer than in M. 



maxima. ,, , 



The greatest antero-posterior measurement of the nacre, parallel 



to the hinge, is about halfway between the hinge and the ventral 



margin, giving the shell a fairly circular outline. 



The colour and markings of the exterior, though extremely 



Proc. Zool. Soc— 1901, Vol. I. No. XXV. 25 



