Lower Helderberg Fauna. 279 



and, becoming detached before gradual deposition covered the 

 organism, were thus lost ; (2) they were of a different composi- 

 tion from the rest of the skeleton ; (3) they did not exist. 



If these summit plates were detached and lost, some trace of 

 them would surely be found either associated with the original 

 fossil or as separate bodies. Of the twenty or more specimens 

 belonging to the large collection from this horizon, each must 

 have possessed many hundred plates, some of which would 

 certainly have been found. 



It seems improbable, however, that one ray of the spicule 

 should have been different chemically from other rays of the 

 same spicule. Furthermore when we consider that the rest of 

 the skeleton is preserved (1) as calcite, (2) as pyrite, (3) as silica, 

 (4) as shaly casts, (5) as ferruginous shadows on shale, it seems 

 incredible that this eccentric ray should have been of such 

 material as to elude all these phases of replacement. Moreover, 

 everything goes to show that the. radiating ridges are an integral 

 part of the original skeleton, and not an infiltration or an 

 impression of the matrix. They are too high and regular to 

 warrant this idea, and, furthermore, they are preserved as pyrite 

 like the rest of the organism, while the matrix is shale, and the 

 filling between the two walls celestite(?). The evidence, there, 

 fore, seems conclusive that the spicular summit plates claimed 

 for the organism did not exist. 



(5) The Tangential Rays. — The same conclusion is inevitable 

 regarding the four tangential rays. A careful examination of all 

 accessible specimens shows no trace of such structure. They can 

 not be the radiating ridges, for these partake of the nature of 

 partitions rather than of spicules, i. e., they are solid, thin and 

 high. Moreover, the orientation of the ridges, which are not 

 longitudinal and horizontal, respectively, as the spicular rays are 

 Slid to be, precludes such an interpretation. The only features 

 that bear resemblance to the missing spicules are the short spines 

 which project, one for each rhomb, from the intersections of the 

 radial ridges. If these are the tangential rays, there is no evi- 

 dence of the three other rays and the summit plate. Further, if 

 this is such a ray it has become disconnected from the " radial 

 arm," and joined to the ridge with which it should have no 

 organic union. 



