ISODICTTA. 1 61 



submitting a portion of it to microscopical examina- 

 tion I found that its structural characters could not 

 be assimilated with those of any other known species 

 of Isodictya. 



" The oscula dispersed on its rugged and uneven 

 surface are scarcely perceptible with the aid of a lens 

 of about two inches' focus, and no indications of the 

 pores could be detected even in portions of the dermal 

 membrane immersed in Canada balsam. The best 

 specific characters are those obtained from the dermal 

 membrane, the tension spicula of which are so equably 

 arranged on some parts as to closely resemble a 

 reticulation, while in other parts they are unmis- 

 takably dispersed. Their form is the same as those 

 of the skeleton, regularly aouate, and they are basally 

 spinous. Their length is -g^ inch and their diameter 

 -gXST inch, while those of the skeleton are Y2T ^t^c^ in 

 length and arsT i'lch in diameter. The retentive 

 bihamate and bidentate spicula are few in number, 

 especially the latter form,, and they are so minute as 

 to require a power of about 400 linear to render them 

 distinct to the eye while in situ ; and it is a remark- 

 able character that the two forms are as nearly as 

 possible of the same length, each of them varying 

 •from 7-5-0 to Yooo inch. The bidentate equi-anchorates 

 are comparatively stout in their proportions, while 

 the bihamates are remarkably slender. This equality 

 in the length of the two forms is very unusual, the 

 bihamate forms being most frequently twice the length 

 of the anchorate ones. Both forms are sparingly dis- 

 persed on the interstitial membranes, and the bidentate 

 equi-anchorates especially so." 



VOL. IV. 1] 



