REPORT ON THE PTEROPODA. 33 



Family II. Clionopsi d.e. 



1855. Clioidea, Gegenbaur (pars), Untersuchungen iiber Pteropoden und Heteropoden, p. 212. 



1859. Pneujywdermonidse, H. and A. Adams (pars), The Genera of Recent Mollusca, vol. ii. p. 613. 



1862. Clionidx, Bronn, Die Klassen und Ordnungen des Thierreichs, Bd. iii. p. 645. 



1873. Clionopsidei, Costa, Pteropodi della Fauna di Napoli, p. 24. 



Characters. — Proboscis very long. Buccal appendages absent. A jaw. A quadri- 

 radiate posterior gill. No lateral gill. Skin not pigmented. 



Description. — Body voluminous, rather transparent, barrel-shaped, a little contracted at 

 both extremities ; posterior extremity rounded ; visceral mass extending to this extremity, 

 forming an axial mass. Skin without pigment, showing generally a great many small 

 spots due to fatty glands ; dorsal spot rounder than in Pneumonoderma. 



Head small, neck rather short ; labial tentacles thick, but short and shrunk at their 

 extremity. 



Foot lying far forward, and without posterior lobe ; anterior lobes like those of 

 Pneumonoderma, connected in all their length ; between these two lobes, on the posterior 

 part of the foot, there is a plicated tubercle. Fins small in proportion to the body. 



Posterior Gill. — Membranous posterior crest rather short, forming a hexagon elongated 

 in the dorso-ventral diameter, and the four ventral and dorsal angles of which each give 

 rise to a small membranous crest which is turned forwards. These crests (the hexagonal 

 and the four radiating ones) show different degrees of specialisation in the different 

 species, and may bear at their base fringes or foldings like those of Pneumonoderma 

 and Spongiobranchtea. 



Proboscis or evaginable anterior portion of the digestive tract (penis, Troschel) 

 excessively long ; it is about three times the length of the body, while in Spongiobranchasa 

 (which of all the other Gymnosomata possesses the longest proboscis), it is not a third of 

 this length. When the proboscis of Clionopsis is invaginated, the horny parts, the radula, 

 jaw, and hook sacs are placed very far from the buccal aperture. 



Radula always with median tooth (tricuspid) in the adult. 



Hook-sacs (whose true signification Krohn was the first to make known ') very short, 

 possessing strong hooks, rather numerous, short, of nearly uniform length, slightly bent 

 and mostly arranged in a bunch. 



1 Beitriige ziir Entwickelungsgeschichte der Pteropoden und Heteropoden, p. 40. 



(ZOOL. CHALL. EXP. — PART LVIII. — 1887.) Mm m 5 



