54 
It is shown in Morris’ upper figure/ which is regarded hy us as 
representing the typical E. curdatnm. 
In making comparisons with 3Ielea(jrina (PI. XXVI, figs. 1-3), a dis- 
tinction is drawn lietween a hyssal sinus and a byssal notch. The sinus is the 
channel, in the right valve, situated immediately in front of the umho, 
leading obliquely downwards and inwards to the interior. The notch is the 
aperture, larger or smaller, caused by an inflection of the anterior valve 
margin. Poth are present in all small species examined, such as M. siiyillata, 
Reeve, or 31. vulgaris, Schum, and in small specimens of 31. margaritifera, 
Linn. (PI. XXVI, figs. 2 and 3), and its var. Cummingii, Reeve, hut in fully- 
grown shells of 31. maxima it appears to be filled up hy a thickening of the 
test of both valves. On the whole, it may he said that a notch is present 
in either delicate and thin-shelled sj^ecies, or the younger conditions of those 
forms which in maturity become heavy and thick-shelled ; through it is 
protruded the hyssus, and when developed to any size, the foot also. 
In all individuals of Eurydesma we have examined, both large and 
small, the valves are entirely closed both anterior and posterior to the umhones, 
leaving no aperture for the protrusion of either byssns or foot. Mr. C. 
Hedley tells us he was informed hy Thursday Island pearlers that these large 
shells lie on their sides without hyssal attachment. The structure of 
Eurydesma leads us to infer that it may have existed in a similar manner. 
It could not have progressed like Pectrn and its allies, in which Dr. II. T. 
Jackson says, “ crawling is effected hy dragging the shell still lying on its 
right side, the foot being extended through a special notch in the right valve. 
Prom studies of Pecten I am led to believe that a notch is a feature caused 
hy the exttuision of the foot quite as readily as hy the existence of a byssns.”^ 
Morris described a “ byssiferous canal passing out of the umhones at the 
margin of the shell.”® But in the upper figure of his illustration — a right 
valve — this is shown as a groove extending across the area ol)liqnely from 
anterior to posterior. It is also to he seen in Dana’s illustration of E. sacculus, 
.McCoy/ and ])ossihly in Johnston’s figure of E. cordata, Morris.^ Lana 
writes : — In one fine specimen, in which the beak of one valve is broken 
^ Morris in Strzelecki — Op cit., PI. XII. 
.Tackson — Mem. Boston Soc. Nat. Hist., 1890, IV, No. 8, p. .828. 
’ Morris in Strzelecki — Op. cit., p. 275. 
* Dana in Wilkes — Op. cit., PI. VII, fig 8c. 
‘ Johnston — Syst. Acc. Geol. Tas., PI. XVII, fig. 2. 
