FORMS OF SHELL IN 15IVALVES. 
41 
Sympiiynote or Connate shells are those in which the valves are 
united together along the dorsal margin, not oidy by the ligament 
hut by continuous shell growth, thus forming a shell practically 
composed of a single piece. This is a character found more especially 
in Anodonta, bnt is confined to the 
juvenile stages of growth, at least in 
British specimens, the testaceous 
connection of the valves sooner or 
later becoming ruptured and broken 
by the opening and closing of the 
shell, and the connate peculiarity 
thus lost. Mr. Lsaac Lea who so long 
and thoroughly studied the Naiads 
at one time attached great importance to this peculiarity and even 
proposed to divide them into two chief groups, according as they 
were or were not possessed of this character. 
Fig. ^S. — A Symphynote or Connate 
Ijivalve. 
Anodonta cygnca (L.), young, pos- 
terior end, 
Grand Junction Canal, Brentford, 
Collected by Mrs. Skilton, 
Showing the testaceous junction of the 
two valves on the posterior dorsal margin. 
Alate or “winged” .shells are sometimes .seen in some of the 
varieties of Anodonta ; the aim or “wings ” being formed by a com- 
pressed upwai'd extension of the posterior dorsal borders, sometimes 
exhibited in a very marked manner. 
This peculiarity is like the symphy- 
notic, a characteristic, more e.specially 
affecting the juvenile life of species, 
at least in British specimens, 
gradually becoming less pronounced 
and striking as the animal advances 
in age and growth. The Symphynote 
and Alate characters are often comhined together in the same 
individual, but this is not ueces.sarily always the case. 
When the two valves fit accurately together at the margins, and 
appear to hermetically close the shell, as in 
Sphwrium corneum and other species, the 
shells are called Close. If, however, owing 
to the margins of the .shell not exactly co- 
inciding with each other in shape or outline, 
the valves do not fit accurately together, 
and more or less visibly open .spaces are left 
between the margins of the two valves for the protrusion of the 
Fig. 99. — An Alate Bivalve. 
Anodonta anatina (L.), young, right valve, 
Canal, Apperley Bridge, Yorks., 
Collected by Mr. J. A. Hargreaves. 
Fig. 100. —A “Close” Bivalve. 
Sphepriiim palliduju Gray, 
posterior end, X 2, 
River Foss, Blue Bridge, York, 
Coll, by Rev. W. C. Hey, M. A. 
