61 
single valve of the Iridina exotica, a very rare koomviii. 
genus, of which Lamarck gives but that one spe- Nat. Hist. 
cies. Also the families Chamacea, Tridacnea, 
and part of the Mytilacea. Of the Chamacea, 
is a specimen of the Diceras arietina, the only 
known species of this genus, and hitherto not 
found as a recent shell; and various species of 
Chama. Thte second Order, of this Class, Con- 
chifera unimusculosa, begins with the family 
Tridacnea, comprehending the genera Tridacna 
and Hippopus. Some shells of the species Tri¬ 
dacna gigas, are the largest that are known ; 
they are said to have been found of 500 pounds 
weight. Part of the genus Modiola, (Mytiius. 
Lin.) of the family of the Mytilacea, are con¬ 
tained in this compartment. 
(Table 12.) Modiolae continued ; and the 
genera Mytiius (the Muscle) and Pinna. 
(Table 13.) The family Malleacea, and 
part of the Pectenida ; including under the for¬ 
mer, the genera Crenatula (rather rare shells), 
Perna, Malleus, (the two last classed as Oysters 
byLinneus) x4vicula (Mytiius. Lin.; and Melea- 
grina. The Meleagrina margaritifera, (Mytiius 
margaritiferus. Lin.) affords valuable pearls, and 
the inside is formed of the beautiful substance, 
known by the name of Mother of Pearl. Under 
the Pectenida, are included the genera, Pedum, 
(containing only one species, P. spondyloideum, 
Ostrea spondyloidea. Gmel.) Lima, Plagiostoma 
(fossil) 
