FORMS OF SHELL IN BIVALVES. 
51 
The Concentric Zones, occasionally occurring in some species, 
sometimes offer a striking contrast of 
colours, especially amongst the species 
of the genus Sphierium. These compara- 
tively broad and usually bright yellow 
zonal markings mostly occur only at the 
free margins of the valves, but at times 
they alternate at regular intervals with 
the normal greenish-brown groundcolour 
of the shell, and have a pretty effect. 
In Unio and Anodonta these concentric zonal markings are generally 
darker in colour than the ground tint of the shell, in some cases 
being quite black ; they represent according to Clessin the winter 
growth of the shell, and therefore indicate the age of the animal. 
In Form, Bivalves are Oblong, when their length, or distance 
between the anterior and posterior margins, is gi-eater than the 
width, or distance from the dorsal to the ventral margin, as in Unlo 
pktorum, Splucrium pallidum, Plsidium milium, Qtc,.] and termed 
Transverse, when the greatest length of the shell is in the opposite 
direction as in Dreksensia. The ordinary differences of form amongst 
onr bivalves are not numerous and may be broadly classed as 
Oval when the shell is of an oval form, as 
in Spka’rium rivicola (Leach), Pisidium 
pusillum (Gmelin), etc. 
Fig. 119. — Example: Sphcerium rhkola (Leach), left \’alve, X 1?, canal, Far Cotton, 
Northampton, collected by Mr. W. D. Crick, F.G.S. 
Rounded when the shell has a somewhat circular 
outline, as in Pisidium nitidum Jenyns. 
Fig. 120. — Example: Pisidium nitidum Jenyns, right valve, X 8, Hackfall, near Ripon, 
Yorkshire, collected by Miss Emily K. Harrison. 
Oblong when the shell has its greatest length 
from the anterior to the posterior margins, 
as in Unio pictorum (h.), Pisidium milium 
Held., l^pluvrium jmllidum Gray, etc. 
Fig. 121. — Example: Spharrium pallidum (»ray, right valve, X 2, canal, Accrington, 
1862, collected by Mr. R. Wigglesworth, 
Fig. 118. — Bivalve with Concen- 
tric Markings. 
Sphcerium corneu7n var. zniiata 
Garn., right valve, X 2, 
River Trent, Marnham, Notts., 
Collected by Mr. W. A. Gain. 
