65 
In the fresh and perfect mollusc the wedge-shaped area is not filled 
with uniting fibres, hut remains open, its sides often l)eeoming encrusted 
with small marine growths. A resilium, ratlier than a ligament, from our 
point of view, exists in Meleagrina, or wliat may, perhaps, he described as 
a “ retiring resilium ” — a resilium the functional area of whicli always keeps 
pace and always remains coincident with the line of valve articulation, the 
upper or dorsal portions becoming atrophied aad functionless. 
Eurydesma differs from this arrangement in ])ossessiiig an entirely 
concealed or true resilium and a transversely elongated resilifer, in which 
the entire resilium is contained. The valves do not i:a])e along the dorsal 
margin, hut are tightly closed. 
Both genera possess a well-marked hyssal sinus, anterior to the 
umbones in Meleagrina, and also in Eferia, hut posterior in Eurydesma. 
A ])yssal notch is also present in young individuals of the larger 
Meleagrinee, and in the small species, hut is wanting in Earydesma . 
Mantle retractor scars are present in both Meleagrina and Eteria on 
the one hand and in Eurydesma on the other, but there is a very marked and 
peculiar difference between the two former genera and Eurydesma . In 
Meleagrma,fn\([ Pteria the scars are situated anteriorly to the large adductor 
scar, but in Eurydesma they are on the posterior side of the adductor scar. 
The umbonal scars so well marked in M. maxima, var. Cumiugii, and in 
Eteria macroptera, and which hold tlie general visceral mass in position, are 
not developed in Eurydesma, unless a homologue exists in the deepish pit at 
the dorso-posterior end of the Z-shaped line of scars, 'the shifting of tlie 
retractor scars from the anterior to the posterior is very remarkable 
in this genus. 
The remaining features in whicli the two genera resemble one another 
are (a) the plain pallial lines ; (i) inequality of the valves, arising more from 
structural details, and far less from mere ventricosity of the valves in 
Eurydesma than in Meleagrina ; (c) the presence in Meleagrina of a series 
of delicate undulations along the ventral edge of the hinge-plate, best seen in 
M. maxima—\\\h condition in Eurydesma is represented by a line of small 
pits on the posterior side. 
(2) Maceoyella. — In internal structure Eurydesma possesses practically 
the same affinity to Maceoyella as it does to Meleagrina. In the former there 
