FORMS OF SHELL IN BIVALVES. 
Fig. 124. — Example: A>iodonta anatina var. co»t/>lnnata Rossm., 
Gumfrieston, near Tenby, collected by Mr. F. Walker. 
Reniform when the shell is oblong, hut constricted near the centre 
of the ventral margin, forming a kidney-shaped outline. 
Triangul.vr when the shell has a 
somewhat triangular outline, as 
in Dreht^ensid jmhpnorphd 
(Palla.s). 
Fig. 122. — Example: Dreissensia />o/ymLy7-J>/ia Baker’s Dock, Stourport, collected 
by Mr. J. W. Williams. 
Trapezoidal or Surriiomboidal when the 
shell is somewhat (piadrilateral in outline 
and the opposite sides though somewhat 
straight, are not parallel, as in ^plucrinm 
Idcustre (Mull.). 
Fig. 123. — E.vample: Spheerium (Mull, j, right valve, X 3, pond, Sandal, Yorks., 
collected by Mr. Joseph Hcbden. 
Co.MPRESSED when the valves are ilattened or compressed, the 
ventral margin forming a very acute angle, as in Anodontd 
dndthid (L.), Spluvriuin lacnstre var. hrochouiaiid Bonrg., etc. 
Fig. 12.5. — Example : Unio inari^ariti/er var. sinuata f.am., right valve. 
Loch Awe, .\rgyleshire, collected by Mr. Alex. Somerville, B.Sc., F.L.S. 
