CHAP. XXII.] MOLLUSCA. 531 



Orde,r IV.—NUCLEO-BRANCHIATA. 



These are oceanic, swimming molluscs, of a delicate texture. 

 They are found in all warm seas, and range back to the Lower 

 Silurian epoch. There are only two families. 



Family 41.— FIROLID^. (2 Genera, 33 Species living, 1 

 fossil.) 



Family 42.— ATLANTIC^. (5 Genera, 22 Species living, 

 159 fossU.) 



Class.— PTEROPODA. 



These are swimming, oceanic mollusca, inhabiting both Arctic, 

 Temperate, and Tropical seas. The three families have each a 

 wide distribution in all the great oceans. They range back to 

 the Silurian period. 



Family 1.— HYALEID^. (9 Genera, 52 Species living, 

 95 fossil.) 



Family 2.— LIMACINID^. (4 Genera, 19 Species Hving, 

 fossil.) 



Family 3.— CLIONIDJE. (4 Genera, 14 Species living, 

 fossU.) 



