MANUAL OF NATURAL HISTORY. 123 



gills in pairs ; abdomen voluminous ; shell 

 symmetrical, calcareous, globular or elongate- 

 conical, furnished with spiny processes. 



2. FAMILY. Subulate-Pteropods (Tripteridse), Body 



elongate, cylindrical, fore part with two large 

 lateral wings united below to a flat central 

 portion ; shell sub-cylindrical, calcareous, sub- 

 angular near the mouth, ending in an acute 

 point separated from the anterior cavity by a 

 transverse septum ; tip often deciduous in the 

 adult. 



3. FAMILY. Slipper -shaped Pteropods (Cymbu- 



liidse). Animal globular or ovate ; fins two, 

 on each side of mouth, with a small interme- 

 diate lobe ; shell cartilaginous, slipper-shaped, 

 rarely wanting. 



4. FAMILY. Spiral-Pteropods (Limacinidse). Body 



spiral, fins elongate, rounded, united at their 

 base by an intermediate lobe bearing an oper- 

 culum ; mantle large, open in front ; shell 

 spiral, usually sinistral, and produced on the 

 collumellar side; operculum ^stinct, spiral 

 vitreous. 



II. ORDER. NAKED-PTEEOPODS (Gymnosomata). 

 Body naked, without any shell ; head distinct ; 

 fins two or four, at junction of head and body, a cen- 

 tral intermediate lobe ; gills exterior ; shell none. 

 1. FAMILY. Olios (Clionidse). Body fusiform ; head 

 with a series of conical prominences on each 

 side ; wings two, with an intermediate foot- 

 like appendage. G 2 



