STKES : VARIATION IN EECENT MOLLUSCA. 



265 



through the list of genera or species in which scalariformity has 

 been noticed, but it may be stated generally that, in most well-known 

 land or fresh-water shells, it has been observed, while in marine 

 shells it is less common. Indeed, all distortions are less common in 

 marine shells, and where present they are chiefly recorded in those 

 species living on the shore near high-water mark. 



Mention may perhaps be made here of the remarkable specimen of 

 Bijthinia tentaculata recorded by Mr. Smith,' which had a very 

 depressed spire and was almost ' Bulloid ' in shape. 



Sometimes the mollusc, after forming the lip in a normal manner, 

 continues to grow and forms a second lip. This has been observed in 

 Selix, Clausilia,'^ etc., while it should be remembered that a somewhat 

 similar occurrence is usual in Acroptychia, Leucoptychia, etc. The 

 suggestion has been made that a rainy season stimulated development 

 and caused the growth to continue. Occasionally a second lip is 

 formed in the mouth at an angle to the first, or irregular growth 

 may continue after the mouth is formed.^ 



In Clansilia and Cylindrella specimens have been found with two 

 apertures, sometimes at an angle to one another, sometimes back to 

 back. Two explanations* have been given: either the shell may be 

 fractured and the animal may make use of the fracture and form 

 a new mouth, or it may be that some obstacle becomes wedged in 

 the old mouth and the animal gnaws its way out in another spot, and 

 forms the new mouth there. 



Rissoa, again, has been observed* to form a new mouth if the old 

 one be broken. Recently a specimen of Helix 2)omatia has been 

 noticed with the mouth distorted and partly closed.^ The curious 

 addendum to the shell of an xstivating Aticylus noticed by NordenskiokU 

 hardly comes within my scope at the moment. 



Zimncea, also, may be deformed by notches in the edge of the 

 mouth, and in some specimens I examined I found that several, which 

 at first sight were normal, had really been so notched, but that 

 subsequently the animal recovered itself and filled up the notch by 

 decrees. The occurrence has been attributed to Hydra.^ 



Beyrich ^ observed clefts in the columella of some species of recent 

 and fossil Natica, and Martens '" has recorded the same in Buccinum, 



• Journ. Conch., vol. v, p. 315. 



2 Westerlund: Xachrbl. Deutsch. Malak. G«s., 1875, p. 84. Clessin: Mai. Blatt., 



A'ol. XX, p. 58. 



3 Baudon: Jomn. Conchyl., vol. xxxii, p. 320. Iselson : Journ. Conch., vol. xv, 



p. 80. 



* Ci. Boettger: Nachrbl. Deutsch. Malak. G«s., vol. vi, p. 98. Gibbons: Quart. 



Journ. Conch., vol. i, p. 340. 

 5 Dautzenbers; : Feuille Xat., vol. xxiii, p. 30. 

 « Kobelt: Xa'chrbl. Deutsch. Malak. Ges., 190-1, p. 125. 

 " Zool. Anz., vol. xxvi, p. 59. 



8 Cf. Brot: Ann. Soc. Mai. Belg., vol. xii, p. 48. Sykes: Journ. Make, vol. iii, 



p. 34. 



9 S.B. Ges. Naturf. Berlin, 1883, pp. 3, 45. 

 10 Ibid., 1889, p. 8. 



