PLATE LXXXIX. 321 



former as the basal membrane has taken an impression 

 of the cellular dermis of the plant, presenting to the 

 eye when mounted in Canada balsam the appearance 

 of a very regular transparent reticulation which might 

 readily deceive an observer into the belief that it was 

 a natural character of the basal membrane. 



The form of the skeleton spicula in D. constrictus is 

 nearly allied to those of this species ; but they differ 

 greatly in size, those of the former being to the latter 

 as eight to three in length ; while in the latter the re- 

 tentive bihamate ones are at least three times as large 

 as those in D. constrictus. 



The skeleton spicula of D. cavernula are of the same 

 size as in D. simularis, but they are stouter in their 

 proportions and are fusiform, while in the latter they 

 are slender and purely acuate, and no tricurvate tension 

 spicula could be found in D. cavernula. The bihamate 

 retentive spicula in the two species closely resemble 

 each other in size and form. 



The nearest structural alliance to D. simularis exists 

 in D. copiosus, in which, with few exceptions, we find 

 the same forms of spicula. But here again, although 

 the skeleton spicula are similar in size, they differ in 

 being distinctly fusiform ; and the tricurvate tension 

 spicula in D. simularis are more slender in their pro- 

 portions and very much longer, being as five to two of 

 those of D. copiosus. 



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