MALACOZOA. GASTEROPODA. NUD1BRANCHIATA. 195 



of the foot, protrudes far beyond the last papillae. It can also 

 creep or repose at the surface of the water, with its back below. 

 When reposing it assumes an elliptical form, contracts the an- 

 terior antennae, but leaves the upper extended. In a vessel 

 with sea-water, it has a habit of reposing for hours at the sur- 

 face with the back beneath, the foot contracted to an elliptical 

 form, the oval tentacula half-contracted and curved backwards. 

 It also frequently creeps out of the water, and reposes in the 

 air. 



FAMILY II. TRITONIINA. 



Body oval, oblong, or elongated, with the branchiae 

 in the form of arbuscules or papillae disposed along the 

 sides, two tentacula retractile into a kind of sunk sheath ; 

 a membranous veil over the mouth ; terminations of the 

 genital and alimentary organs distant, on the right side. 



GENUS 1. TRITONIA. 



Body oval or oblong, convex above, flattened and hav- 

 ing a broad muscular disk or foot beneath; the head 

 broad, with two retractile, pectinate or tufted tentacula; a 

 broad semicircular veil over the mouth, which is furnished 

 with two lateral thin-edged and denticulate teeth ; bran- 

 chiae in the form of tufts or plumes arranged symmetri- 

 cally on the two sides of the body. 



1. Tritonia arborescens. Arborescent Tritonia. 



Body oblong, convex above, smooth ; with four feathery 

 appendages on the margin of the veil over the mouth ; tenta- 

 cula conical, transversely striated ; branchiae in distinct decom- 

 posed tufts, six on each side, decreasing in size from the head 

 backwards ; foot narrow ; the colour above dusky, of the bran- 

 chiae darker, of the lower parts whitish. Length about an inch, 

 height nearly half the length. 



An individual brought up by the lines from deep water off 

 Aberdeen, presented to me in the Winter of 1841. 



Tritonia arborescens. Cuv. Mem. 28. PI. 1. f. 8, 9, 10. Tritonia 

 arborescens. Flem. Brit. Anira. 284. Tritonia arborescens. Johnst. 

 Ann. of Nat. Hist. i. 1 15. 



