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ancient world. Among the estinct fossil Cephalopoda ■we 

 shall find one family only allied to the cuttle fishes, nameh"i 

 the beleranitidoe, consisting of about 100 species; but of 

 the testaceous cephalopoda, the shells or casts of about 

 1200 species are found in the primiiry and secondary sedi- 

 mentary rocks. This argument if taken alone, may not be 

 conclusive, since the extinct species are distributed in suc- 

 cession throughout these immense geological periods ; but 

 it must be coupled with the fact that not only the species 

 of these testaceans are extinct, but also the genera and the 

 families. Some 900 species belong to the extinct family of 

 theammonitidae, and between 500 and 600 to the one genus 

 ammonites, a fossil furni, familiar to every one, and the 

 whole of this great group, with the single exception of the 

 nautilus, has been extinct ever since the formation of the 

 chalk ; at the most half-a-dozen species of the nautilus 

 are in the land of the living, to Olustrate the tale of their 

 ancient relatives. The Cephalopoda constitute the most 

 important and interesting chapter in the history of the 

 molluscs, but it is not to the present purpose to enlarge 

 upon it. 



The second class is that of the Gasteropoda, or stomach- 

 footed, so called because they crawl upon a broad foot placed 

 under their bodies ; whoever has seen a snail in motion will 

 at once comprehend this structure. As I shall speak of the 

 Gasteropoda more in detail, we will pass on to the third 

 class, called the Pteropoda or wing-footed— in plain English 

 they are furnished with fins. This is a group of small ani- 

 mals, and they differ from all the other molluscs, inasmuch 

 as they inhabit the wide ocean, where they swim by means 

 of their fins in shoals miles in extent ; one species, the Clio 

 Borealis, inhabits the Arctic Ocean in such masses as to 

 form a staple article of food to the whale. The Pteropoda 

 are partly testaceous and partly not, and some of those 

 which have shells are only partially covered by them, leav- 

 ing the fins free to act. 



Ilere arc a few of the Pteropoda, shown on a scale large 



