284' A MONOGRAPH OF THE 



or osculum, at the distal end of the branch ; occasionally 

 this orifice is lateral, and then its thin margin is slightly 

 elevated above the general surface. When the branches 

 touch each other in the course of their growth, they inos- 

 culate, and assume an irregularly latticed form. The 

 dermal reticulation is a very beautiful object, the areas are 

 most frequently triangular, and it often occurs that six 

 triangles, each formed of three spicula, are packed together, 

 an angle of each meeting the others at a central point, and 

 their opposite basal lines forming a very regular hexagon, 

 and this singular arrangement obtains to a considerable 

 extent, but it is occasionally confused by the occurrence of 

 regular or irregular quadrangular areas. The sides of the 

 areas never appear to exceed the length of a single spicu- 

 lum ; and the spicula rather exceed in diameter those of the 

 skeleton. The primary and secondary lines of the skeleton 

 are both unispicular, and the arrangement is very regular 

 and exceedingly delicate and beautiful, the interstices being 

 more regularly quadrangular than is usual in an Isodictya. 

 The spicula are small and equable in size, and are rather 

 stout in proportion to their length ; the tension spicula of 

 the interstitial membranes are less in lensrth than those of 

 the skeleton, and very much more slender ; they are not 

 numerous, and occur in detached patches, containing three 

 or four only. 



8. Isodictya McAndrewii, Bowerbank. 



Sponge. Massive, sessile, surface irregular, minutely hispid. 

 Oscnla large, fistulous. Pores inconspicuous. Dermal 

 membrane pellucid, furnished with a uniserial net- 

 work of stout acerate spicula. Skeleton. Primary 

 lines rarely more than bi- or trispiculous, secondary 

 lines unispiculous. Spicula acerate, stout, and short. 

 Geminules oval, furnished abundantly with minute 

 acerate (?) spicula, radiating in the direction of lines 

 from the centre to the circumference. 



