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EVENINGS AT THE MICROSCOPE. 



and free from cavities. This calcareous skeleton is 

 -encased in a gelatinous flesh, similar to and continuous 

 with that of the stalk. 



This is the smallest kind, the head being about -=-Vth 

 of an inch in height. 



Considerable modifications are found to exist in the 

 details of each form, in the relative proportions which the 

 parts bear to each other, and so forth ; so that two forms, 

 which in their extreme conditions widely differ, mutually 

 •approach, and appear to run into each other. This is 

 the case with the present, and with the form which I 

 -will now show you. 



P. tridens is much larger than any of the other forms, 



the movable head being about -^V ^h 

 of an inch in length, and the wdiole 

 organ about -^th of an inch. This 

 may be considered as essentially 

 P. trip/iylla, modified by the 

 blades being greatly drawn out in 

 length, and at the same time ren- 

 dered quite slender, so that they 

 may be called pins; they meet only 

 at the points, where they often 

 cross ; the spaces between the basal 

 parts being open. The inner edges 

 of these are notched with teeth as 

 in P. triphyUa t of which those near 

 the tips are larger, and cut into 

 subordinate teeth of exquisite 

 minuteness. 



We have here an opportunity 

 of seeing that the oval or square 

 markings, which are thickly placed 

 throughout the calcareous sub- 

 stance of the blades, are certainly cavities in it; for in 

 dhose examples in which the pins, which are very brittle, 



HEAD OF PEDICELXARIA 

 TBIDENS. 



