EXPLANATION OF THE PLATES. 835 



than the sixteen-celled ; I have observed in these, besides 

 that figured, the following kinds of arrangement : 



1 + 5 + 10 + 16 

 1 + 6 + 10 + 15 

 1 + 5 4- 11 + 15 

 1 + 6+11 + 14 (spiral.) 



5 + 11 + 16 



6 + 10 + 16 (sub-spiral.) 



7 + 10+15 (eUiptioal.) 



I have never met with specimens with sixty-four cells 

 in this species, but Nageli, (1, c, t. v, b, 1, g,) figures a 

 sixty-four-celled specimen, arranged 2-|-7-t^l2-|--.194-24, 

 in the scarcely specifically distinct P. Boryanum. 



