1888.] MICKOSCOPICAL JOURNAL. 9 



is the type, and the knowledge of this one will lead to that of the other 

 four. 



If, on the other hand, a shell is present, then there are 8 genera to be con- 

 sidered, namely ; Difflugia^ HyalospJienia. ^nadrzila, Nebela^ Heleopera^ 

 Arcella, Centropyxis, CochliopodiJim. 



Look now to the character of the shell. Is it formed of large sand-grains? 

 It is a Dijflugla or a Centropyxis^ and the position of the so-called mouth 

 or opening, whether inclined or not inclined, will determine between them. 

 Is it formed of square plates? It is a ^uadrnla. Are the markings round 

 or oval, like a mass of eggs, or round and rod-like, intermingled like a piece 

 of mosaic ? It is a Nebela. Finely punctate, the dots resolving into hexagons 

 under a higher power? It is an Arcella. 4, 5, or 6 sided, the sides formed 

 of beads? It is an Heleopera. Is the shell smooth or scalloped at the edges 

 while being oval in form ? It is an Hyalosphenia. Lastly, does the shell in- 

 vest the creature like a membrane, and exhibit at the circumference dots in 

 cross-lines like those of a diatom? It is a Cochliopodhun. Thus we have 

 disposed of the 13 genera belonging to the Lobose group. 



And the Filose division may be mastered with equal ease. Here are only 

 9 genera instead of 33 to begin with, and the generic characteristics are dis- 

 tinct and readily remembered. The list is as follows : Pamphagus^ Pseu- 

 dodiffugia^ Cyphoderia^ Campasctts^ Euglypha^ Placocista. Assulina^ 

 Triizenia^ Sphenoderia. Of these, three may be recognized at once : — Cy- 

 phoderia by its shape, that of a retort ; Etiglypha by its large hexagonal plates 

 and serrated mouth ; and Assulina by its scales, like those on a butterfly's 

 wing. Trine?na has an inclined pouch-like shell. In Sphertoderia the 

 plates are round or oval, arranged in alternating rows, and often intersecting. 

 Campa.sctis has a couple of horns to the shell. Placocista is a thorny spheno- 

 deria. Pamphagiis has a flexible investing membrane, often folded in one 

 part or another ; and Psezidodijplugia is a difflugia with filose instead of lobose 

 pseudopods. 



This will make it clear, I think, that the study of the rhizopods is not so 

 formidable as a glance at Leidy's huge volume would incline the tvro to imag- 

 ine. Of course you will understand that the key suggested, which has been 

 drawn from Leidy's illustrations, is bv no means exhaustive. It will not 

 settle all forms, but it will enable the beginner to determine the great ma- 

 jority ; will give him such a start that he can afterwards push the matter by 

 the aid of Leidy's splendid monograph. It is the first step which costs, and 

 there is nothing that will more discourage the student than to be met at the 

 outset by an elaborate series of descriptions, or even by an involved analysis 

 of the genera. What is needed is that the work should be simplified to the 

 last degree, the most salient features alone being pointed out, so that he may 

 be encouraged to proceed by feeling that he is making progress. This has 

 been my object, and I shall hope to convince you, by means of diagrams, that 

 it is possible to become acquainted with the different genera in a very easy 

 way. 



But before concluding the 23Hper let me say a word or two about a few other 

 points connected with this study. 



First, about the size of these objects. They are very minute, and the plates 

 in Leidy's work will be almost sure to mislead, even though one use the 

 power he appends to the descriptions. Thus some specimens of Difflugia 

 pyriforjnis are only j^nth inch in length; oi nebula^ -^-^Sh.'^ oi Arcella 

 vulgaris^ "sio^^^ ' *^^ Assulina, ^-^^-^\}ci ; of Trinema, Y^^oyth. Of these, 

 Difflugia may be met with as large as j'^th inch, but the others will not run 

 above .jTioth- And the breadth, except in the case of Arcella^ is m?ich less. 

 Amoeba varies still more, from ^^^yth to g-gVo*^^ inch. I repeat, therefore, that 



