SEA-FISHERIES LABORATORY. B43 
the left, however, is slightly more convex than the right 
(see Text-fig. 1). Both species have suborbicular valves, 
and these are marked by plications so that the outer 
surface has a number of ribs arising near the umbo. The 
ribs are not present in old specimens of P. maaimus on 
the areas immediately adjoining the umbos. 
2. = 
Fie. 1. Diagrammatic sections of P. opercularis and P. maximus to show 
shape of valves in natural position. 
The number of plications appears to be constant 
throughout life. no new ones arising by bifurcation or 
interposition, and there are fewer in P. maaimus than in 
P. opercularis. The average numbers ean be obtained 
from the following table given by Davenport (41) for the 
shells of P. opereularis from three localities : — 
Ribs. Off Eddystone. Trish Sea. Firth of Forth. 
14 ee Oroe 0 00 Os 07 
15 Den O29) OF. 3) ORD SO 
16 tay bed o7 PAT ee Bese 63 12-4 % 
Ni 195 364% 152 24:8 % 154 30:3% 
18 182 340% HIG) ao Ge 164 32°3% 
i) 66 12:3% IGS DaOee JOSE OIOZ 
20 Mee Bed) Us Oe PAD eg) OF 
21 0 Bee Sila A ARB; OF 
22 OLA O%, LL OY 
536 614 508 
Davenport has also given the relation of the dorgo- 
ventral diameter to the antero-posterior diameter, for 
1,657 shells of P. opercularis from these same localities. 
The results show that the smallest shells are from off the 
Kddystone hghthouse, the largest from the Firth of Forth, 
and the intermediate ones from Port Krin in the Irish Sea. 
