120 LECTURES ON MOLLUSCA. 



lines almost straight, and gaping at each end, are called Adrana, and 

 found in tropical seas. The animal of Yoldia is very active, and leaps 

 very far on its bent foot. The .group Portlandia has an irregularly 

 swollen shell, truncated at the side. Neilo has a similarly-shaped 

 shell, but not nacreous, and with the cartilage external. The mantle- 

 edge is double, and furnished with flaps. It is found living in New 

 Zealand, but fossil in Patagonia. Solenella is a similar shell from 

 Chili, but shaped like.Sanguinolaria, nacreous within, and with part 

 of the anterior tooth remaining undivided, as in Hacrodon and Nuci- 

 nella. 



Family TRIGONIADJE. 



The Trigonia race make their appearance in the secondary rocks, 

 and abound as far as the cretaceous age ; but in the tertiary series 

 they have not yet been found. They linger however along with 

 other old forms, in the Australian seas, presenting us with shells and 

 animals of surpassing beauty. They have long, sharply-bent, pointed 

 feet, like the Cockles, with which they can take surprising leaps. But 

 they resemble the Arks in having the mantle freely open, the foot-sole 

 crenulated, and the gills united/ They are almost entirely nacreous 

 within, and strongly sculptured outside. The hinge has 2-1 very 

 large, deeply furrowed teeth. In many strata, the shell has entirely 

 perished, leaving very characteristic internal casts, called " horse- 

 heads" by the quarry men of the Portland oolite. Myoplioria has a 

 similar shell, but less sculptured. Ascinus makes its appearance in 

 the Upper Silurian, with small, smooth teeth. Similar shells have 

 been described as Mactra, Isocardia, Anodontopsis , Anatina and Dola- 

 bra. Lyrodesma is the earliest form in this family, with several radi- 

 ating teeth, striated across. Verticordia is a small group from the 

 newer tertiaries, and still living; with thin, nacreous, I^cma-shaped 

 shells, with two Unioid teeth in each valve. The Eocene Hippagus 

 has a similar shell without teeth. This family combines many of the 

 characters of Nucula, Castalia, and Cardium. 



Family AVICULID^:. (Wing-Shells, Pearl and Hammer Oysters.) 



This extensive family of living and extinct forms are remarkable for 

 the microscopic structure of the shells, as shown by Dr. W. B. Car- 

 penter, (in the British Association Reports, "before quoted.) The out- 

 side portion consists of large prisms; which in transparent young 

 shells can be detected with a single glass, and in the old decaying 

 shells of Pinna easily break up into needle-like fragments, resembling 

 Arragonite. These have been formed by rows of simple shells, some- 

 times of different colors, piled one over the other. The fragments of 

 the great Inocerami from the cretaceous rocks have the aspect of fossil 

 wood. The same structure is found in the floats of Belemnites. The 

 inside of the valves consists of true pearls, the beautiful iridescence of 

 which is caused by very finely wrinkled skins, with layers of shell be- 

 tween. After the shell has been dissolved in acid, arid the wrinkles 

 flattened out, the iridescence ceases. Many of the fossil forms have 

 shells intermediate in form between Avicula and Pecten; but their 



