60 
not appear to be the case, for tlie muscle adherent to this scar, like that in the 
nmbonal cavity, does not })erform the functions of an adductor. Dr. R. T. 
Jackson, who has made an exhaustive study of the Pteriidse and Pectinidm, 
states that Pteria [Avicula) possesses only one adductor in the adult — the 
large subcentral muscle.^ In the young state the mollusc was dimyarian, 
the anterior adductor disappearing Avith.age.” 
In Eurydesma there is a similar more or less Z-shaped line of retractor 
])its, but somewhat differently arranged. The pits commence above with a 
single or double transversely elongated well-marked pit or pits, rather larger 
tlian those succeeding, situated immediately below and near the end 
of the hinge-plate in each valve ; these form the head line of the Z. The 
line of pits then descends either vertically or oblicjiiely for a short distance, 
and finally turns anteriorly in a long tail of pits. These are, with little 
doubt, the scars of the reti-acf or muscles of the mantle and the viscera, although 
proceeding from below the jjosterior end of the articulus rather than from the 
more anterior j^osition of the umbonal cavity. This change places the retractor 
scars in a much more dorsal position in relation to the adductor in Eurydesma 
than it does in Meleagrina and its living allies ; a study of certain of our 
figures (PI. XX, fig. 3 ; PI. XXI, figs. 4 and 5; PI. XXIV, figs. 1 and 2) 
will render this clear. 
In some specimens a single detached ])it occurs beyond the head line 
of the Z-shaped figure, and more posteriorly ; the details of the arrangement 
vary to a slight extent. 
A semicircular line of retractor scars also occurs in Maccoyella^ but 
as in ]\Ieleagrina, is wholly anterior to the adductor scar. 
These scars have received the attention of previous writers. Morris 
figured on the interior of the right valve a slightly curved line of unconnecb'd 
round depressions immediately below the umbonal cavity, the “small muscular 
impressions.” ‘ In E. saccnlus, Dana, instead of this semicircle of depressions, 
there are, at about the same position, two only, half an inch apart, and 
supero-posteriorly arc four smaller pit-like depressions forming a curved line 
* Jackson — Mem. Boston Soc. Nat. Hist., 1890, IV, No- 8, p. 282. 
* l)all in Zittel — Text-book Pal. (Eastman’s Eel.), 1890, I, p. 370. 
•Etheridge — Mem. Geol. Survey N. S. Wales, Pal. No. 11, 1902, PI. Ill, fig. 5. 
Morris in Strzelecki — Loc. cit., p. 276, PI. XII, upper figure. 
