SPONGES, GRAPTOLITES, COEALS. 63 



Ischadites.l 



to exclude the passage of water from the exterior to the interior cavity of the organ- 

 ism, is shown by the fact that one of the horizontal spicnlar rays projects from under- 

 neath the distant angle of each of the plates and extends over the outer surface of 

 the plate in front, thus clearly preventing a close-fitting union at the margins, and, 

 further, the ridges, which characterize the outer surface of the casts of specimens, 

 are produced by the infilling of the matrix in the interspaces between the margins 

 of the plates. 



"These summit- or head-plates appear to have been connected by a somewhat 

 narrow neck to the horizontal rays of the spicules at the central point of junction 

 with these and the vertical rays, as the horizontal rays appear to be independent 

 except at their central junction. As a rule, the head-plates are seldom preserved in 

 situ. 



" The surface of the fossil immediately beneath the rhomboidal spicular plates 

 is divided into minute oblong rectangular areas by vertical and concentric lines. 

 These lines are formed by the apposition of the horizontal spicular arms or rays. 

 The spicules, in addition to the head-plate, consist of five rays ; four extended in a 

 horizontal direction, at right angles to each other, whilst the fifth extends from the 

 junction of the four with the summit-plate towards the interior of the organism, and 

 thus at right angles to the horizontal rays. The spicular rays are circular in trans- 

 verse section, thickest at their central point of junction with each other and the 

 head-plate, and they gradually taper to bluntly-pointed extremities [In Ischadites 

 iowensis they are needle-shaped]. Canals present in the interior of the rays. The 

 vertical or entering ray appears to be the longest, the lateral rays are subequal, 

 whilst the distal ray, or that pointing to the summit of the specimen, seems to be 

 longer than the opposite or proximal ray. 



" The four horizontal rays are so arranged that each ray extends towards one of 

 the angles of the head-plate of the spicule. Thus one ray, the proximal, points to 

 the basal nucleus, and its opposite, the distal, to the summit. This distal ray in the 

 basal portion of the organism frequently projects beyond the margin of the spicular 

 head, and overlies the head-plate of the spicule immediately in front or above it. 



" The vertical rays of the spicules, which extend at right angles to the summit- 

 plates and the horizontal rays, are only seen when the interior of the specimens is 

 exposed by fracture or by section. They appear as delicate, gradually tapering 

 shafts, the extremities of which are pointed, and reach about half way to the center 

 of the interior cavity, where they terminate freely. An interior plate or integument 

 corresponding to that in Receptaculites has not been observed. 



" The genus Ischadites itself has, by several writers, been regarded as identical 



