OF MOLLUSCA. 93 



into minute wing-like processes, and at the terminal point is subcir- 

 cularly scalloped out, the lateral margins forming usually four, some- 

 times three or four, very short caudal fillets, sometimes either rudi- 

 mentary or quite obsolete. The single respiratory plume, branching 

 from right to left, has 35 to 45 or more long slender pectinations ; the 

 neck is simple and free from all lappets. — Clark. 



Lacuna pallidula, L. vincta, and L. crassior have no jaws ; the teeth 

 are nearly alike in the three species, in from 45 to 60 cross rows ; in 

 L. crassior the central tooth is higher than in the others. The oto- 

 lites are circular and simple. — Alder. 



In Lacuna puteolus the lateral appendages of the operculigerous 

 lobes are large, but the caudal filament very short, and often obso- 

 lete. L. pallidula has the lateral wing-like extension with two or 

 four very short caudal lobes. — Clark. 



The foot of Lacuna pallidula has a longitudinal fissure, and the 

 progression is alternate as in Littorina ; branchial plume single, 

 of 35 to 45 or more long slender pectinations ; neck simple, without 

 lappets. The verge elongate, simple, under the right tentaculum. 

 Operculigerous lobe bifid behind. — Clark. 



The central tooth of Lacuna canalis (fig. 51) is almost hexagonal ; 

 cutting edge with five pointed lobes ; first lateral tooth behind deeply 

 notched between the two processes, the upper margin five- to six- 

 Fig. 51. — Teeth of Lacuna canalis. 



toothed ; the second slender, subclavate, five-toothed ; the third claw- 

 like, the anterior margin somewhat toothed before the tip. — Loven. 



The Lacuna feed on Fucus, eating the surface of the frond or 

 stem, often forming holes through the former and deep holes in the 

 latter, into which many specimens often crowd themselves. They 

 are also often found crowded together in the crevices among the 

 roots of the plant. The animal has no filament on the end of the 

 foot. 



The LacuncB live among the roots of Laminaria and other marine 

 plants, attached to stones and shells below the low tide level, but 

 the Fuci are often found, dragged by the storms, on sandy beaches. 



Fam. XL TRUNCATELLID^E. 



Shell spiral ; whorls gradually enlarging. Apex acute, deciduous. 

 Aperture ovate. Peristome continuous. Muzzle broadly two-lobed. 



