ISODICTTA. 159 



way very irregular. The primary lines are rarely 

 symmetrically disposed and the number of the spicula 

 of which they are constructed varies considerably, and 

 this, combined with the irregularity of the structure 

 and mode of disposition of the secondary portions of 

 the skeleton lines, presents the whole in so confused a 

 condition as to render the isodictyal characters in 

 many parts very difficult of detection, and the abundant 

 presence of numerous small, acerate, interstitial tension 

 spicula greatly increases the confusion of the structures; 

 but a careful observation of the structual peculiarities of 

 the primary lines of the skeleton decidedly determines 

 their isodictyal character. The skeleton- spicula are 

 constant in their structural characters, but they vary to 

 a large extent in their diameters and in their mode of 

 spination, some of them being sparingly but entirely 

 spinous, while others were furnished with a few minute 

 spines at the base and apex only. 



" The tension spicula of the interstitial membranes 

 are not equally abundant in all parts of the structures ; 

 in some parts they abound, while in others they are 

 very few in number. 



" The only two species of British sponges with which 

 this species might possibly be confounded are Isodictya 

 lurida and I. rugosa. In the first of these two species 

 - both the skeleton and the dermal tension spicula are 

 nearly twice as long as those of I. incequalis, the length 

 of a fully developed spiculum being y^-g- inch, while 

 that of I. lurida is -^ inch, and the latter is also 

 much less spinous than the former, and about the 

 same inequality in size exists in the tension spicula 

 of the dermal membrane. The skeleton- spicula of 

 I. rugosa very closely resemble those of L incequalis 



