QUADRULA IRREGULARIS. 135 



chitinous material but of some combination of calcium. 

 " After studying the works of Lagerheim," Penard 

 says, " I myself experimented on various examples of 

 Quadrula belonging to the forms glolmlosa (from Lake 

 Geneva) and disco ides. My procedure consisted in 

 isolating an individual, and then, under the microscope, 

 touching it with a minute drop of sulphuric acid. At 

 the moment when the acid touched the shell this was 

 seen to be instantaneously dissolved, but there remained 

 a fine chitinous film which preserved the contour of the 

 shell, and the outlines of the square plates. At the 

 same time there was formed often a bubble of gas, 

 which filled the shell ; this most probably arose from 

 the reaction of the acid on the shell contents, for 

 in the case of clean and empty shells no such bubble 

 was produced. If the shell thus acted upon were 

 pushed forward, by lightly touching the cover-glass, 

 the soft chitinous film would be folded in upon itself 

 and become like a thread, showing no trace of any 

 solid elements. If, on the other hand, it was exposed 

 to the flame of a lamp for an instant, the film would 

 dissolve completely in boiling acid, leaving no trace." 



Lagerheim's experiments, the author concludes, 

 establish the fact that the Quadrula test whether dis- 

 coides, monensis, or other is covered with calcareous 

 plates, or plates formed of a combination of calcium, 

 placed side by side upon a very fine chitinous pellicle. 



The examples from Knutsford Moor belonged to 

 the form discoides. They were exceedingly abundant 

 throughout the gathering, and were the smallest 

 examples of the species we have met with, besides 

 exhibiting great uniformity of structure. 



The variability of Q. irregularis is indicated by a 

 form of the var. globulosa described by Penard (' Archiv. 

 fiir Protistenkunde,' 1903, p. 261) occurring in a 

 gathering from Spitzbergen in which the minute test, 

 whilst showing no compression, had a tendency to 

 become ovate or sacciform, and irregular in contour, the 



