THE PALEONTOLOGY OF MINNESOTA. 



[Ischadites. 



u The structure consists of spicules of various dimensions, regularly arranged in 

 vertical and oblique rows, and held in position by the interlocking of their summit- 

 plates and horizontal rays. Head-plates of the spicules delicate structures with 

 smooth, flattened outer surfaces, thickest in the central portion where they connect 

 with the horizontal rays, and gradually diminishing towards the margins, which are 

 very thin. They have a generally rhomboidal outline, but in some parts of the speci- 

 men two of the sides of the rhomboids are not uniformly straight, but have a slight 

 curve, which gives the plates the form of a sector of a circle. Another modification 

 is frequently, if not invariably, present in the spicular-plates of the basal portion, 

 which have their angles, or those directed away from the basal nucleus, either trun- 

 cate or with a slight notch, from which one of the horizontal rays projects and 

 extends nearly to the center of the plate immediately in front. The plates forming 

 the basal nucleus are also more elongated than any others. The plates near the 

 nucleus, as well as those of the nucleus itself, are relatively small, but they 

 quickly increase in size towards the zonal area, where they attain their great- 

 est dimensions (2 to 4 mm.); they then gradually diminish in size towards the 

 summit, and the smallest plates .surrounding the summit-aperture are scarcely dis- 

 tinguishable without a lens, measuring from .25 to .4 mm. in width, or about one- 

 tenth of the diameter of the zonal plates. 



" Head-plates arranged in regular spiral curves which, starting in opposite direc- 

 tions from the basal nucleus and extending to the summit, give to the surface the 

 exact appearance of the engine-turned case of a watch. Each rhomboidal plate is 

 so arranged that one of its angles points to the basal nucleus, and its opposite angle 

 to the summit of the specimen, whilst the other angles are lateral, so that the distal 

 angle of one plate is in contact with the proximal angle of the plate immediately 

 in front of it. Thus vertical lines extending from the base to summit would pass 

 through the proximal and distal angles of the plates, whilst concentric lines would 

 pass through the lateral angles'. At the nucleus, or center of the base, there is a 

 series of eight minute spicules with diamond-shaped head-plates, which are so 

 arranged as to form a star-shaped figure, the distal angles of each plate representing 

 one of the rays of the star, and a line connecting the lateral angles would trace a 

 small circle, with the proximal angles of the plates for its center. 



''As a rule the margins of the plates appear to fit closely and evenly to each 

 other, but in some cases the upper or front margins seem to be slightly elevated as if 

 they imbricated over the lower or hind margins of the spicular plates immediately 

 in front, and left a small intermediate space now filled with the matrix. That the 

 plates, or at least those of the lower portion of the organism, did not fit so closely as 



