Dr. Wallich on the Ehizopods. 459 



esting group of testaceous Amoebans, as consisting " of delicate, 

 transparent and colourless chitinoid membrane, without trace 

 of definite structure." In ^^ Hyalosphenia tincta " and "JT. 

 pa-pilio " the tests are said to be pale yellow. I have never to 

 my knowledge fallen in with Hyalosphenia, but am neverthe- 

 less able to attest the occurrence of equally delicate chitinoid 

 tests as occurring in one of the commonest forms when 

 developed under favouring conditions, namely Arcella vul- 

 garis. 



But Prof. Leidy's view appears improbable for another and 

 more substantial reason. It is this, that although the plates 

 of QuadruJa are described as above in the letterpress, there 

 is not in any of the six figures (nos. 20 to 25) of plate xxiv. 

 of his work, representing different views and forms of the 

 kind of tests referred to, a single plate that even conveys the 

 idea of having been intended to represent a true rectangular 

 figure. On the contrary, with the exception of perhaps five 

 or six plates out of some two hundred and fifty which nearly 

 answer to the description of being square, all are more or 

 less irregular both in their outlines and angles, and in these 

 respects yield no such evidence of an approach to crystalline 

 form as is to he seen in some of the other tests represented in 

 Prof. Leidy's work as belonging to true Difflugidoi. 



Moreover, nearly all the plates in the figures of Quadrula 

 (pi. xxiv. figs. 20-25) are drawn as having more or less 

 convex external surfaces, another character not met with in any 

 specimens that have fallen under my notice. Indeed, it was 

 the apparent perfect accuracy of the rectangles and the tabular 

 surfaces and very definite margins of the plates in ray speci- 

 mens that led me to suspect their being due, in some degree 

 at least, to crystalline agency, although I was fully alive to 

 the fact that crystallization under ordinary circumstances is 

 interfered with instead of being promoted in presence of 

 colloids. 



In Prof. Leidy's plate x. fig. 26, said to represent a shell of 

 Diffiugia pyriformis, composed " of chitinoid membrane incor- 

 porated with thin siliceous plates^'' some of these are really 

 rectangular; but I should feel inclined to regard them as 

 rectangular rods, as their length as compared with their 

 width in most of the forms exhibiting them varies apparently 

 from 4 to 1, to 10 or 12 to 1, or even more ; the elongated form in 

 such examples being in all likelihood the natural consequence 

 of the siliceous constituent being derived from diatoms, many 

 of which are of similar proportions. Some plates also, on " a 

 shell oi D. arcula, in pi. xv. fig. 36, and in figs. 16, 17, 18, 

 and 19 of the same plate, representing D. lobostoma, are in 



