74 Dynamic Theory. 



branchia. This little animal has a shell on its back and another 011 its 

 belly, in which respect it differs from the clam tribes which have the 

 shells on their flanks. 



The shells of the braehiopod are not hinged, but at the hinge edge, 

 which is the one opposite the mouth, a fleshy peduncle or stem pro- 

 trudes between the edges of the shells or through a hole in one of them, 

 by which the animal is fixed to a rock, mouth up. 



The brachiopods are so called from their two spiral or coiled arm-like 

 feet, which are attached at the sides of the mouth and can be protruded 

 to collect food, and also act as gills. These arms are, in some families, 

 furnished with fringes. Sometimes inside the shell there are loops or 

 coils of calcified matter for the arms to rest in. They have a single 

 nerve ganglion which probably acts to correlate the sense of touch in 

 the arms. White blood circulates in the body cavit}' or ccelom. There 

 are ten families of brachiopods. Two of these families, the Discina and 

 Lingula, have representatives in the Potsdam period, and what is re- 

 markable, one genus or another of each of these families, is to be found 

 in all the great geological periods from that day to this, and they are 

 still extant. New species and genera have constantly arisen as the old 

 ones have died out, but the family type still persists. 



There are two other families represented, the Orthis or Strophomena, 

 and the Rhynchonella families. The former became extinct in the Lias, 

 while some species of the latter still exist. 



Of mollusks proper we have, in the Potsdam, first, Pteropods ( wing- 

 footed animals). They are minute in size, generally in a conical, spiral 

 or two plated dorsal and ventral shell, though sometimes naked. They 

 have a more or less distinct head, on each side of which is a wing-like 

 appendage which operates like a fin, by which the animal swims. They 

 are permanently just what the sea snails are in their larval state, from 

 which it may be inferred that the sea snails are descended from the 

 Pteropods. The Pteropods, under Haeckel's new classification, would 

 come under class Cochlides. 



The next division of Mollusks represented in the upper part of the 

 Potsdam, viz., the Calciferous epoch, is the Gasteropod (or belly foot), 

 so called because he projects his ventral muscle and moves on it some- 

 thing like a foot. This class, and the Pteropods just spoken of, are 

 designated as Cephalates or those having heads. They are divided into 

 two sections: those breathing by means of gills, and those breathing air 

 by means of lungs. Snails and slugs are examples of the latter. The 

 fossil Gasteropods from the Potsdam formations, are, of course, of the 

 gill-bearing section. Inside the shell the animal is enclosed in a man- 

 tle or loose skin, which is sometimes prolonged into a tube or siphon in 

 front. The shell is generally spiral, like that of the snail. In some,, 



