GENUS PLACOCISTA— PLACOCISTA SPINOSA. 223 



limited by zones of minute ellipses, as seen in the figures of pi. XXXVIII. 

 The plates bordering the' mouth do not differ from those elsewhere, so that 

 the margin of the latter is entirely destitute of the tooth-like processes, 

 which form so conspicuous a feature in the species of Euglypha. Mostly, 

 the margin of the mouth is quite even, but often the contiguous oval 

 plates appear to give it a feeble wavy course. 



The lateral borders of the shell extending to the fundus are fringed 

 with awl-shaped spines, variable in number, length, and degree of robust- 

 ness. The spines spring from the acute margin of the shell, or on each 

 side of it, from between the contiguous plates. They are usually in pairs, 

 but are often single, and occasionally a tuft of three may be seen with the 

 others. The root of each spine possesses a minute rounded knob, by which 

 it is movably attached to the shell, so that the spines may be turned in 

 any direction, though usually they diverge outward and upward. 



The sarcode and pseudopods of Placocista are identical in appearance 

 with those of Euglypha. Although I have repeatedly found Placocista 

 spinosa at different seasons and apparently under favorable conditions, I 

 have rarely observed it in an active condition. The animal seems to be 

 among the most shy of its kind. In most instances, the sarcode was 

 retracted within the shell, was completely free from any recognizable food 

 materials, and appeared to be ready to pass into an encysted condition. 

 Though I have certainly seen some scores of living individuals, I do not 

 remember to have observed more than four or five emit their pseudopods, 

 and assume the erect position, as is customary with creatures of the kind. 



The sarcode occupies the interior of the shell to a variable extent, 

 sometimes completely filling it, and at others more or less partially, as 

 represented in the figures of pi. XXXVIII. In reduction, the mass of sar- 

 code undergoes contraction from the sides of the shell, but appears to 

 retain a close connection with the fundus. Rarely have I seen it connected 

 with the interior of the shell by pseudopodal processes, as exemplified in 

 the active individual of fig. 4. 



Most specimens of Placocista spinosa observed appeared to be perfectly 

 free from food contents, presenting the mass of sarcode as a colorless, trans- 

 parent, finely granular protoplasm mingled with variable proportions of 

 darkly outlined oil-like molecules. The greater quantity of the latter, as 

 usual in Euglypha, etc., are accumulated in an intermediate position of the 



