SEELET — ON FOSSIL BIVALVE SHELLS. 
49 
Types with Umhonal and Posterior Teeth. 
Panopsea. 
Margaritina 
•1-2 
•11 
Hippopus 
Tridacna . 
Ensis* . . 
Solen . . 
Cardita . 
Kellia . . 
Isocardia 
Cardium . 
Tellina. . 
Galatea . 
Donax . . 
lunule perforate. 
•11-11 
■1-2 
' ^ ' ^ S ; umbones termi- 
• 1 • 1 ' iial. 
Cultellus 
Machera . 
Tancredia 
Castalia . 
Unio . . . 
Cyprina . 
: S. 
1 • 1 
^ ' 1 S; fromumbotobase 
1 • 1 ' runs an internal rib. 
2^1' 
•2-1' 
•21 
•1-2 
•2-2^ 
•111' 
3^ 1 
131 •I 
Types loith Anterior and other kinds of Teeth. 
1-1' V 
I'l' 
I'^ll' 
(_11)'-1-(1 1)' 
(l-l)l^:l^- V 
I : (1 • 1)1'- I'l 
111111 
II ll^ll 
i • 12 i • • 1 
1 • 1 12 • • 1 
1- 1 „ 
; S. 
1-21 
12 
1 • 2M 
Lucina . . 
Corbis . . 
Cyclas , . 
Cyrena . 
Corbicula 
Trigona . 
Meroe . . 
1 1 
1 •2-1 
L: S. 
1-21 , , 
jT^Tj^; margins creuulated. 
1 ^l 
13 1 
I 1 • 3 • 1 1 
(11)'^3^(11)' 
I I • 3 • 1 ' 
13 
1 !'• 3 
L ; E ; S. 
]Many of the formulae will be found unsatisfactory, no doubt ; for it 
has not been my object either to give a synopsis of bivalve genera 
or of generic characters, but merely to illustrate the features and 
capabilities for distinctive purposes of some remarkable mystical 
structures. Yet I would ask the student who takes some member 
of one of the larger types, say Donax, Tellina, Cardium, or whatever 
it may be, and fails to torture its hinge into agreement with the 
formula, fairly to ask, — What a genus really is ? Unless I mistiike, 
facts are as faulty as the formulfe. This, however, involves the whole 
(juestion of the nature and origin of genera, which will shortly be 
treated in full elsewhere. 
* A form like this is uot iutroduced as a genus, but merely as an illustration of re- 
sults arrived at by adhering to the formulaj rigidly. 
VOL. VII. H 
