CHAP. XXII.] 
MOLLUSCA. 
531 
Order I V. — NUCLEO-BRANCHIA TA . 
These are oceanic, swimming molluscs, of a delicate texture. 
They are found in all warm seas, and range hack to the Lower 
Silurian epoch. There are only two families. 
Family 41. — FIKOLID4E. (2 Genera, 33 Species living, 1 
fossil.) 
Family 42. — ATLANTIDiE. (5 Genera, 22 Species living, 
159 fossil.) 
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. — HYALEIDaE. (9 Genera, 5,2 Species living, 
95 fossil.) 
Family 2. — LIMACINIDiE. (4 Genera, 19 Species living, 0 
fossil.) 
Family 3.— CLIONIDiE. (4 Genera, 14 Species living, 0 
fossil.) 
M M 2 
