A SKETCH OF GEOLOGY. 25 



called the mesial fold. Cardinal area is a synonym for hinge -area, and cardi- 

 nal extremities are the extreme points of the hinge-line. Of all the Mollusca, 

 the Brachiopoda enjoy the greatest range both of climate and depth and time ; 

 they are found in tropical and polar seas. The living species prefer the deep 

 waters to shallow lakes, though some of them are found even here. Of the 

 population of the seas of former ages, they formed a very large part. Their fos- 

 sil remains are found in almost every strata of the whole geological column. 

 They are not only very abundant, but also well preserved, and form the princi- 

 pal treasures of many palaeontological collections. 



LAMELLIBRANCHIATA. The Mollusca of this class are familiar to every one ; 

 they are represented by the oysters, mussels and cockles. The animal is 

 without head, and the shell is bivalve. The valves are attached to the sides 

 of the animal. We have, therefore, in this class, a right and left valve. Plate 

 I, figures 3 and 20, show the left valves. In figure 17, the upper valve is a right 

 one, and the lower a left one. The mouth of the animal is generally directed 

 towards the shorter slope of the shell. The hinge is on the back of the animal 

 and formed by teeth and sockets, together with an elastic ligament. The shells 

 are closed by powerful adductor muscles, but open when the animal relaxes 

 the muscles, or when it is dead. We distinguish in the shells of this class the 

 dorsal, ventral, anterior, and posterior margins. The dorsal margin contains 

 the hinge, whilst the ventral margin is opposite to this. The anterior margin 

 forms the end of the shell on the shorter slope, and the posterior margin the 

 end of the longer slope. 



Plate I, figure 3, is the left valve of Cytherea dione, separate from Cyth- 

 erea, with the following prominent parts : h, the hinge ligament ; d, the umbo ; 

 f, the lunule ; c, cardinal tooth ; tt, lateral teeth ; a, anterior adductor muscle ,- 

 a, posterior adductor muscle ; p, pallial impression ; s,- sinus, occupied by re- 

 tractor of the siphons. The line designated as pallial impression, which in 

 many shells is prominently marked, indicates the place where the mantle is 

 attached to the shell. The presence of a bay or sinus (s), in the line of the 

 pallial impression, proves the animal to have possessed retractile siphons. 

 All the Mollusca belonging to this class live in the water, in rivers, lakes and 

 oceans. They are abundant at the present time, and have been so in all the 

 seas of former ages. Their fossil remains are often extremely well preserved, 

 and add greatly to the beauty of many Palaeozoic collections. 



Within the here described five classes, the different Mollusca are divided into 

 families, genera and species. The naturalists of former centuries were able to 

 give a more precise definition of these terms. They believed that each species 

 was a priori created, and thus by sharply defined lines of distinction, separated 

 from all the other forms, and only related to some of these by accidental sitni- 



GKOL. SUR. 4 



