84 
ROOM XTI. 
Nat. Hist. 
In some, the two leaves of the mantle are united 
together behind, and extended into more or less elong¬ 
ated tubes, as in the genera Artemis , Cytherea, Ve¬ 
nus, and Venerupis, which have three teeth in 
each valve, and an external cartilage; Cyprina, 
Crassina, Pisidium and Cyclas , differ from the former, 
in having no syphonal inflection, bearing a thick peri- 
ostracum; the last two are also only found in fresh 
water. Isocardia is peculiar for its strongly incurved 
umbones, and very oblique teeth. 
Case 10 contains the genera Lucina and Loripes, 
which have an opaque white internal surface to their 
valves, and no syphonal scar, the former having an 
external, and the latter an internal cartilage; also the 
Solens , Psammobia , and Tellina , which are elongated 
and gaping at one or both ends, the last having the 
hinder extremity obliquely twisted. 
Case 11 contains the genera Cardium and Donax , 
called also wedge-shells, from their shape, which have 
only two teeth in each valve, forming a kind of 
cross. Then follow the bivalve shells which have 
no cartilage, the valves being separated from each 
other by means of muscles, placed over the umbones, 
and covered with a thin skin. In Teredo, this skin is 
simple, and the animals line the holes made by them 
with shelly matter, forming a tube; in the genus 
Pholas, this skin is protected by one or more shelly 
plates. This caused the Linnean conchologists to 
consider the former as a univalve, having mistaken the 
tubes for the shell of the animal, and the latter for a 
multivalve shell. 
Case 12 contains the genera which have the lobes 
of the mantle united, and which are peculiar for having 
the cartilage of the hinge inserted in an internal 
cavity, as the genera Mactra and Lutraria, which are 
equivalve, and have cardinal teeth ; My a, Corbula, and 
Pandora , which are inequivalve; and Anatina and 
Mygdala, which have a peculiar piece of shell placed 
on 
