32 SHELL GALLERY. 
FlLIBKANCHIA. (Fig. 25, B.) 
In this group the gills are smooth, with the filaments directed 
downwards, reflected, and connected one with another by inter- 
filamentar ciliated junctions, but the lamella* are not connected. 
The foot is usually provided with a well-developed byssal gland. 
Anomia, Area, Trigonia, and Mytilus belong to this order. 
The family of Anomiidce contains a number of more or less pearly 
shells remarkable for a deep notch or hole in the lower or flat valve 
through which a shelly plug passes, by means of which the animal 
attaches itself to other shells, stones, &c. Anomia mnigmatka is 
found adhering to leaves in mangrove-swamps. 
The Placunidm, sometimes called Window-shells and Saddle- 
Oysters, are very flat pearly shells with a remarkable hinge, which 
consists of two long divergent teeth, like a /\, to which the ligament 
is attached. The species are few in number, and inhabit sandy 
shores of India, China, and North Australia. 
The Arcidw are a family of strong ponderous shells varying much 
in form and sculpture. The animals have a longish pointed foot, 
deeply grooved along the bottom, no labial palpi, and free margins 
to the mantle, which are not prolonged into breathing-siphons. 
Many of the Arks often anchor themselves by means of a strong 
byssus. The shells of this family are usually radiately ridged ; and 
the hinge is composed of a number of teeth arranged along the 
hinge-line, which is generally straight. Area tortuosa, from China, 
has the valves curiously twisted. The section Baroatia is remark- 
able for the coarse fibrous character of the epidermis ; Scapharca for 
its unequal valves ; and Cucullma, from the Indian Ocean, for the 
elevated ridge bounding the posterior muscular impression. Glycy- 
meris (better known as Pectunculus) has the hinge-teeth arranged in 
an arched series, and the shells are more regular in growth than in 
many other forms of Arcidm. 
The TrigoniidcB are one of those families which have all but 
disappeared during our period. Only three or four living species are 
known, whilst more than a hundred fossil forms have been described 
from the Jurassic and Cretaceous formations. Australia, where 
some of the oldest types of animal life persist, furnishes also the 
existing species of Trigonia (Fig. 27). The animals have a long, 
sharply-bent, pointed foot like the Cockles, with which they make 
surprising leaps. The shells are beautifully pearly within, and 
ribbed and noduled exteriorly. 
