92 SYSTEMATIC ARRANGEMENT 



Head of Fossar ambigua proboscidiform, produced. Tentacles 

 filiform, pointed, with an internal frontal lobe ; eyes not prominent, 

 at the outer base of the tentacles. Foot moderate, rounded at each 

 end. Operculum horny, semiorbicular, simple, not spiral. Shell 

 semiglobose, umbilicated. Mouth large, semicircular ; lip toothless, 

 not callous. Umbilicus open, outer lip acute, smooth within. — 

 Philippi. 



Fam. X. LACUNID.E. 



Mantle enclosed, simple-edged. Trunk produced, wrinkled, not 

 retractile. Jaws none. Tongue linear. Tentacles far apart. Gills 

 single, of numerous strands. Operculigerous lobe expanded at the 

 side, and furnished with two beards behmd. Shell solid, spiral, free ; 

 aperture ovate ; inner lip flattened, grooved. Operculum spiral, 

 horny, few-whorled. 



1. Lacuna. 

 Shell conical or subglobose ; spire short ; mouth half-ovate ; inner 

 lip flattened, with a deep umbilical groove. 



1. L. canalis, Wiegm. Arch. 1836, t. 8. f. 5. 



2. L. vincta, Forbes fy Hartley, B. M. t. G.G. f. 4. 



3. L. divaricata, Adams, Gen. Moll. t. 33. f. 6. 



The Lacunce are robust ; the head strong, thick ; eyes nearly 

 sessile ; tentacula long, slender ; male organ at the base of the right 

 tentacula, strong, compressed, thick, long ; operculigerous lobe with 

 a styliform beard on each side ; sole panduriform. Living on Fuci ; 

 on brown Fuci they are green, on red Fuci rose-coloured. The egg- 

 cases are worm-shaped, thick, curved into a semicircle. The young 

 swimming, with a vibrating veil. — Loven, Ind. Moll. Scand. 22. 



Lacuna pallidula is pale flesh-coloured, Rostrum very short ; 

 mouth orbicular, radiately plicated, emitting an elongated cylindrical 

 proboscis. Tentacles lateral, elongate, filiform, tapering, slender, with 

 the eyes on a short swollen tubercle on the outer side of their base ; 

 sides of body simple. Foot oblong, rounded before and behind. Oper- 

 cular disk expanded, with a slight filiform process on each side 

 behind. The foot is quite continuous, sometimes regularly gliding 

 along in walking, and at others alternately moving first one side of 

 the front margin and then the other. They are very impatient when 

 in the water, and desirous of getting out of it, sometimes floating on 

 the surface like Lymnece ; very viscid. The gills are enclosed on the 

 left side of the cavity. These animals differ from Littorince in the 

 shape of the mouth, in having a proboscis, and in the hinder lobe of 

 tin- opercular mantle. The p)roboscis has not been observed by any 

 other malacologist, and I have not the means of verifying the fact at 

 present ; but the absence of jaws renders it probable they should 

 he removed to the former order. 



The operculigerous lobe of Lacuna pallidula is expanded laterally 



