56 LIMNMIDjE. 



This local species sometimes disappears mysteri- 

 ously from localities in which it has been known to 

 exist, reappearing however after a time as plentifully 

 as before. This curious phenomenon has not been 

 satisfactorily accounted for. It is scarcely probable 

 that the animal actually leaves its habitat ; perhaps 

 for some reason it buries itself for a lengthened period 

 in the mud, and thus escapes observation. Moquin- 

 Tandon says that it is a lively animal, being usually 

 on the move, and that it greedily devours the thread- 

 like and tender roots of some of the aquatic plants. 

 The capsules are colourless and transparent, and 

 contain from thirty to forty eggs. The shell of the 

 young is completely covered by the expansion of the 

 mantle. 



Var. 7nucrojiata. — Shell not quite so globular; spire more 

 produced. 



B. Shell not covered externally by the mantle ; 

 spire usually produced. 



2. L. INVOLU'TA,* Thompson. Pl. V. 



" Body dark yellowish-brown, more or less speckled closely and 

 irregularly with flake-white at the sides of the head, tentacles, and 

 foot ; mantle wholly enclosed within the shell, which it lines, a 

 space being usually left between it and the mouth ; no part of the 

 mantle is outside the shell either when the animal is crawling or 

 not, its edges are somewhat thickened ; head very large, broad, 

 and semicircular ; 7tioiith placed below the head-disc and in the 

 middle of it, and extended sideways ; it is armed with a pair 

 of jaws which are disposed transversely ; te?itacles large and tri- 

 angular, with blunt tips ; eyes small, black, seated on the inner 

 base of the tentacles ; foot lanceolate (lance-head shaped), or 



* Rolled inwards (in allusion to the sunken spire). 



