A SKETCH OF GEOLOGY. 21 



line of demarkation between the groups as he organized them, and in each 

 group he found the above stated percentage of recent species ; but other locali- 

 ties of Tertiary rocks showed more than 4 and less than 18 per cent. , and others 

 again more than 20 and less than 35 per cent, of living species among their 

 fossils. The question there and then arose, to which of the groups such strata 

 should be attached ? This question is solved by placing them in those groups 

 with which their fossils show the nearest relationship. 



MOLLUSCA. 



Mollusca form one of the great animal sub-kingdoms ; their name is derived 

 from the Latin word "Mollis," meaning soft, on account of the soft consistency 

 of their body. This name was given to them by the French savant, Cuvier. 

 This sub -kingdom embraces the Mollusca proper and the Molluscoids. The 

 latter include the Polyzoa or Bryozoa and the Ascidians. The Ascidians hav- 

 ing no shell, but instead only a kind of leathery sack, are not, so far, found in 

 a fossil condition. The Bryozoa were formerly classed with the Corals on 

 account of the resemblance of their calcareous support with those forms, but 

 a closer study of the animals themselves proves their nearer relationship with 

 the Mollusca. Inasmuch as the following monograph only treats of the fossil 

 remains of the Mollusca proper, I deem it sufficient to limit my description to 

 the latter. 



The soft condition of the animals of this class makes it necessary to provide 

 them with, a protection in some shape or form. This is given them either by a 

 calcareous envelope called their shell, or by a leathery sack or mantle surround- 

 ing the body. This mantle is possessed by every Mollusca, whether it is pro- 

 vided with a shell or not; but the mantle of those without shell is more 

 leathery and better adapted for protection than the mantle of the shell-bearers. 

 The shells are either of one, two or several pieces, and are accordingly called 

 univalves, bivalves and multivalves. The common garden snail belongs to the 

 first, the river mussel to the second, and the Chitons to the last group. Some 

 of the Mollusca have a regular head furnished with eyes, tentacles, and a mouth 

 with jaws and teeth. This class is called Encephala. The balance, having no 

 head, are known as Acephala. The first group is divided into Cephalopoda, 

 Gasteropoda and Pteropoda; the second into Brachiopoda and Lamellibran- 

 chiata. 



CEPHALOPODA, meaning head-footed, so named because their arms or feet are 

 arranged around their mouth. To this class belong the Nautilus, the Argo- 

 naut and the cuttle-fish of the present fauna, and the Ammonites, Cereatites 

 and Belemnites of former ages. The bodies of most of them are symmetrical, 

 that is, both halves of their bodies are identical in parts and size; their loco- 



