216 proceedings of the academy of [1883. 



General Anatomy. 



In the following description I will first consider the anatomy of 

 A. fiuviatilis as a basis, for the anatomy of this is tolerably well 

 known from the papers of Carl Yogt * and Moquin-Tandon.-^ The 

 first paper is short and incomplete, containing at the same time 

 many mistakes, while the latter, unfortunately, is without plates. 

 On A. lacustrU no paper has as yet appeared, as far as I know. 



The shell of A. fluviatilis is much larger than that of A. lacus- 

 fris. In both species the form is that of a depressed cone and of 

 a dirty brown color. In A. Jiuviatilis it is said ^ that the shell is 

 wound to the left. I have never as yet seen a shell of A. Jiuvia- 

 tilis which was in the least unsymmetrical, for the apex of all the 

 specimens that I have examined lay in the median line, onl}'^ rolled 

 a little backwards. 



In A. lacudris^ however, the apex of the shell is wound slightly 

 to the right, and this character has been considered sufficient to 

 place this foi-m in a separate genus, that of Acroloxus (Beck, 

 1831), or Vellitia (Gray, 1840), which, however, is not generally 

 accepted. 



The opening of the shell {apertura) is oval in both species ; in 

 A. lacustris, howcA'cr, it is a much longer oval than in A. fluvia- 

 tilis. 



The shell contains such a quantity of conchyolin, that if it be 

 thrown into an acid and left there until all the carbonate of lime 

 be dissolved away, the organic framework of conchyolin remains 

 perfect and the form unchanged. 



If a piece of this be placed under the microscope a large number 

 of the siliceous cases of diatomes are seen. This is easily 

 explained : the diatomes are found in large quantities on the 

 objects on which the Ancyli are found, and as they are so small, 

 they can easily pass between the mantle and the shell and then 

 become covered b}^ a layer of mother-of-pearl or nacre which is 

 secreted by the external surface of the mantle and by which the 

 shell grows in thickness. This process of imbedding diatomes in 

 nature is similar to that effected artificially by the Chinese, when 

 they place their little leaden images between the mantle and the 



' Bemerkungen iiber den Bau der Ancylus fluviatilis. Archiv fiir Anat. 

 und Physiol. (Miiller), 1841. 

 * Recher. anat. physiol. s. I'Ancyle, etc. 

 ' C. Claus, Grundziige d. Zoologie, Marburg, 1880-82, and others. 



