59 
the impressions are “ much like those in Meleagrina In E. ellipficnm, 
Waagen, “there is one rather larger but very flat muscular impression of a 
rather roundish shape not far from the centre of the valve, being, however, 
yet considerably posterior in its position from a line drawn from the apex to 
the middle of the ventral margin,” ^ 
9. Visceral Retractor Scars. — On the inuer surface of the valves 
of Meleugrhia, maxima, Jameson, is a series of small scars forming a 
roughly sigmoidal line extending either from below the anterior end of the 
resilifer, or from an umhonal pit, downwards to the anteiior end of the 
adductor scar. These scars in linear series are said by Tryon to be those of 
the mantle retractor muscles.^ This has been confirmed by dissection. The 
terminal scars of the series, whether below- the resilifer or in the umhonal 
cavity and in front of the adductor scars, are always larger than the 
intermediate impressions. 
This line is more or less clearly shown in all species of Meteagrina 
we have examined, hut the umhonal pit is plainer and deeper in some 
forms {e.g., M. 'margaritifera , var, Cuniingii) than in others ; it is mnch 
more marked in Rteria macroptera, Lamk. Dissection shows that this 2>it 
is simply the apical impression of the retractor series, and plays a most 
important i>art in retaining in position the general viscera of the mollusc; 
wdien the animal is entirely removed from the valves the muscle fibres related 
to this particular scar remain erect — -those adherent to the intermediate scars 
are flat, hut tlieir fibres may be seen streaming out through the snhstance of 
the mantle. This umhonal muscle is said by Tryon to l)e the anteiior 
adductor, and is given as a family cliaracter of the Pteriidae (Aviculidm) — 
“ posterior muscular impression large, suhcentral, anterior small within the 
umho”^; as, however, it does not extend from valve to valve, but gives 
attachment as already described, it is difficult to see the applicability of the 
term adductor to it. 
The ventral terminal scar of this scries, like the superior one, alw^ays 
larger than the intermediate scars, is regarded by Mr. L. G. Seurat as the 
anterior in Meteagrina.'^ Prom an examination of the Pearl Oyster this does 
* Waageii — Op. cit., p. 141. 
* Tryon — Struct, and Syst. Conch., 1884, 111, p. 271. 
^ Tryon — Loc. cit., p. 270. 
* Seurat — L’Huitre perlifere, nacre et perles, p. 8, fig. 1. 
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