32 



SHELL GALLERY. 



Cases 

 137-153. 



Cases 

 137-138. 



Case 138, 



Cases 

 139-141. 



Case 141. 



FlLIBEANCHIA. (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 lamellge are not connected. 

 The foot is usually provided with a well-developed byssal gland. 

 Anoniia, Area, Trigonia, Mijtilus, Pteria { = Avicula), Spondi/his, 

 and Pecten belong to this order. 



The family of AnomUdm 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 cmiigmatica is 

 found adhering to leaves in mangrove-swamps. 



The PlacunidcB, 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 f\, to which the ligament 

 is attached. The species are few in number, and inhabit sandy 

 shores of India, China, and North Australia. 



The ArcidcB 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 Barhafia is remark- 

 able for the coarse fibrous character of the periostracum ; Scapliarca 

 for its unequal valves ; and CucuUma, 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 Arcidce. 



The family TrigoniidcB is one of those which have all but disap- 

 peared 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 animallife 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 take surprising leaps. The 

 shells are beautifully pearly within, and ribbed and noduled exteriorly. 



