MARINE MOLLUSCA OF THE UNITED STATES. 11 



Body elongate ; back with a central, transparent line over the 

 keel of the shell. Fins rhombic. Sessile arms slender ; dorsal 

 pair rounded externally, with a slight fin on the upper part; the 

 second, third, and fourth pairs finned on the outer side nearl^^ the 

 whole length. Tentacular arms with six large hooks. Shell dark- 

 brown, lanceolate, pennate, with a short central keel above and 

 ridge beneath, thin. 



Gulf Stream, Indian Ocean, Xew Zealand. 



Genus ONYCHIA, Lesueur. 

 Journ. Philad. Acad., i. 98, 1821 ; ii. 29G, 1822. 



In this genus the body is red and spotted ; the tentacular arms 

 scarcely enlarged at the ends. Like most other genera of this 

 faniilj', and other pelagic forms, it is crepuscular, darting along 

 the surface of the ocean toward nightfall, and prej'ing upon small 

 fishes, floating Crustacea and acalei)hffi that swim near the surface. 



1. 0. CARDIOPTERA, Perou. Fig. 6. 



(Loltgo.) Voy. Atlas, t. GO, f. 5. 1804. 

 Scpiola cardioptera, Lesueur, Journ. Philad. Acad., ii. 100. 1821. 

 Onychia caribma, Lesueur, ibid., ii. 98, t. 9, f. 1, 2. 1821. 

 Onychoteutlds Leachii, Ferussac, Cepli. Ac'-t. Onycli., t. 5, f. 4, 7. 



Bod}- large, oblong, narrowed and prolonged behind. Sessile 

 arms unequal, relative lengtlis 3, 2, 4, 1 ; cups in two alternating 

 lines. Shell rather broad, sides rounded. 



Soutltern Atlantic Ocean; Oidf of Mexico. 



Genus OMMASTREPHES, Orbigny. • 

 Moll. Viv. et Fos., i. 413. 1845. 



In this genus the sessile arms are conical, subulate, upper and 

 lower quadrangular, the others triangular, compressed, unequal ; 

 relative lengths 3, 2, 1, 4; the cups are very oblique, flesh}', dis- 

 tinct ; the rings oblique and toothed. Tentacular arms not re- 

 tractile, short, strong, thiclc, with a sliglit longitudinal ridge, 

 scarcely enlarged at the end, webbed at the tip. 



Living in the high seas in large troops, nocturnal ; the food of 

 cetacea and pelagic birds. The sailors call them " sea-arrows" or 

 " flying-squids," from their habit of leaping out of the water, often 

 to such a height as to fall on the decks of vessels. 



