50 
MARKINGS IN BIVALVES. 
layer of soft mud ; in disturbed waters and on a different bottom the 
shells are usually duller in colour and often eroded at the umbones. 
Diversity of Markings is not a striking character of British 
bivalves, the most remarkable ex- 
ample being furnished by the angular 
or zig-zag markings on the more 
exposed surface of the shell of 
Dreist^eiK^id puli/morj)/i((, the acutely 
angulated podial ridge forming a 
sharply defined boundary line, sepa- 
rating the distinctly ornamented 
posterior and general surface of the shell, from the plain and uniformly 
coloured anterior or podial area, from which the angular markings 
are (pute absent. 
The Radiate markings, so noticeable at times on the Naiads, are 
variable in colour, but are usually of some shade of yellow, brown, or 
green. 'I’hey arise in the umbonal region, and are directed towards 
the free margin, but generally those rays which occupy the posterior 
Fk;. 1 17.--l*ivalve with Radiate Markings. 
Anoilonta anaihia var. rad'iata JcfTr., riglil vaUc, 
River Foss, Rliie J’ridgc, near York, collected hy Rev. W. C'. Hey, M.y\. 
part of the shell, are more strongly coloured and more distinctly 
defined than those on the anterior part, which in the genera Unln 
and Anodoutd is often more or less deeiily imbedded in the sandy 
or muddy bottom of the lake or stream. These radiate and zig-zag 
markings are generally most vivid and distinct in immature shells, 
usually becoming more or less indistinct and obliterated by age. 
Fig. 116. — lUvalve with Zig-zag 
Markings. 
Dfi'issciisia po'yjnorpha (Pallas), 
Baker’s Dock, Stourport, 
Collected by Mr. J. W. Williams. 
