48 
THE LIGAMENT IN BIVALVES. 
with the bihd laterals of the right valve, and thus offering a marked 
contrast to the denticles of the Unmtes, in which this arrangement 
is reversed. Dr. Jeffreys, Reeve, and many other authors, have fallen 
into error on this point. 
It should perhaps be mentioned that many continental and American 
conchologists regard the antero-lateral lamella) of the Unioiies as 
cardinal denticles, considering the antero-lateral lamella) to be absent. 
The Ligament is an nncalcified chitinons part of the .shell usually 
Ku;. 114. — Hivalvt* with Kxtenial Ligament. 
Anodontu amitina (L.), 
Sandy Ited of River Vare, IJramerton Wood-end, near Norwich, 
Collected by Rev. S. Spencer Pearce, M.A. 
/. e.vternal ligament ; u. umbo ; g. gape. 
attached to riilges along the dorsal margin, posterior to the nmhones, 
and uniting the valves along the ni)i)er or dor.sal margin. It is narrow, 
feeble and membranous, when existent anterior to the umbones, and is 
only functionally elHcient and powerful posteriorly. It is large and 
strong in A iKidoiifd, etc., but small in .some marine genera which 
have a largo internal cartilage between the 
v.ilves. It is strong, bi-ownish, convex, aaid 
very flexible during the life of the animal, 
but on its death becomes dry and brittle, 
thongb regaining .some of its elasticity on 
immersion in water. The ligament is usually 
external, as in Anodanta, but is sometimes 
internal and lodged in a ligamental pit in 
each valve, as in Ihud !<sfdis/d polymorplm. 
When the valves arc closed the elastic fibres 
forming the external ligament are stretched, 
but when the ligament is internal the fibres 
are compres.sed, the effect in both cases 
being the .same, causing the valves to oi)en 
as .soon as the adductors are rela.xed. The 
ligament is said to consist of two layers - 
an outer one, corre.sponding to the epiconch of the shell, and an 
I' lc. llo. — Hivalve with In- 
ternal Ligament. 
Dycissi’Hsia po/y/uor/>/ia ( Pallas), 
( iloiicesler'Canal, Stroud, 
Collected by Mr. K. J. ICIliott. 
/. ligamental pit ; a. ad. ant. 
adductor .scar; /.ad. post, ad- 
ductor scar; p.r. post, pedal and 
byssal retractor scar ; /./. pallial 
line. 
