139 



Class PTEROPODA. 



Plates 473, 474. 



The Mollusca of this class are of simple structure, and are 

 characterised externally by a pair of wing-like flappers, 

 which are used as organs of locomotion in the sea. Although 

 the animal parts are alike in the various genera and species, 

 the shells are as different in form as any shells can be from 

 each other. They range from an imitation of bivalve shells 

 with the valves soldered together, and a simple tubular form 

 to ordinary spiral and symmetrical Nautilus-like forms. 



Genus HYAL^EA, Lamarck. 



Testa quasi bivalvis, tenuis, lateraliter appendiculata, sym- 

 metrica ; apice nonspirali, apertura angusta, labio marginato, 

 plerumque in rostrum producto. — -The shell of an Hyalsea 

 resembles that of a symmetrical bivalve with upper and 

 lower valves soldered together, and the lower one produced 

 beyond the upper, mostly in form of a beak. The apex (the 

 lower part of the figures) is produced, sometimes curved, but 

 never spiral. Near the apex are lateral appendages or pro- 

 duced points on each side. 



Sjpecies. 



1. TRiDENTATA, f. 5, FovsJcal. — Globosa succinea; rostro 

 subquadratim producto; apice acuto ; appendicibus binis 

 distantibus. — The form is rounded, without the apex and 

 rostrum, about as long as broad. The two lateral tlu-ee- 

 cornered appendages near the apex are broad and distant, 

 and the apex sharply produced. 



