A SKETCH OF GEOLOGY. 23 



2, a, the apex ; C, c and d, sutures ; a to f, the spire ; g, the aperture. In 

 figure 14 of the same plate, "a" is the posterior canal, "C" the anterior canal, 

 and "d" the outer lip of the aperture. Each full turn of the spiral is called a 

 whorl. The axis (see Plate I, figure 23), ("a"), around which the whorls are 

 coiled, is either solid or it is hollow, as may be seen in Plate I, figure 10. In 

 this case the shell is called perforated or umbellicated. Nearly all the shells 

 are dextral, or right-handed ; others are sinistral, or left-handed ; see Plate I, 

 figures 14 and 15. The first is dextral, the second sinistral. In a few species 

 the shell is regularly sinistral ; for instance, in Clausilia, while among those 

 with dextral spires, sometimes sinistral aberrations are found. The last turn 

 of the shell is called the body- whorl, and the aperture is generally closed by 

 an operculum. 



The aperture is either entire, as in Plate I, figures 10 and 21, or drawn out 

 or produced into a canal, see Plate I, figures 14 and 15. Species having shells 

 with an entire aperture, are generally vegetable feeders, whilst, on the other 

 hand, the siphonated shells belong to carnivorous families. The Gasteropoda 

 are found all over the world on land, in rivers and fresh- water lakes, and in 

 the different oceans. Their fossil representatives run through all the different 

 geological strata, from the lowest Silurian up to recent formation of the 

 Pliocene. 



PTEROPODA. (Wing-footed.) Their locomotion is accomplished by means 

 of a pair of large fins attached to the sides of their head (see Plate I, figure 4). 

 Their shells are univalve, but of very different forms. Plate I, figure 5, shows 

 the fossil shell of one genera of Pteropoda, a Conularia. Animals of this class 

 are very abundant in all our oceans, and furnish food for a good many inhabi- 

 tants of the sea. In former ages they lived in all the oceans from the Silurian 

 age to those just preceding the now existing ones. 



BRACHIOPODA. The Brachiopoda are bivalve shell-fish, which differ from 

 the ordinary mussels, cockles, etc., in being always equal sided, but never 

 quite equal-valve. Their forms are symmetrical, and so commonly resemble 

 antique lamps that they were called lampades, or lamp shells, by the old natu- 

 ralists. The hole which, in a lamp, admits the wick, serves, in the shell, for 

 the passage of the pedicle by which it is attached to submarine objects. The 

 valves of the Brachiopoda are respectively dorsal and ventral. The ventral 

 valve is usually the larger, and has a prominent beak, by which it is attached 

 or through which the organ of adhesion, the pedicle, passes. The valves are 

 articulated by two curved teeth in the ventral valve, which are received by 

 sockets of the dorsal valve ; some genera and species are not provided with such 

 hinges. In both the articulated and unarticulated Brachiopods, the valves are 

 opened and closed by strong muscles, whose places of attachment are seen in 



