274 Report of the State Geologist. 



At one time the tubes were probably normal to the two walls, 

 but in places they now lie nearly tangent to them. This dis- 

 placement, as well as the absence of the lower basal pole from 

 the field of the specimen, is apparently due to a skewing of the 

 walls as the result of pressure. For the same reason, the spines 

 above referred to may have projected longitudinally instead of 

 laterally. This is shown by the fact that the spines in this speci- 

 men bisect the obtuse angles, and that the long diagonal of the 

 rhombus is normally directed horizontally instead of laterally. 



Where the tubes join the outer wall, their diameter is about 

 one-half that of the rhomboidal pit,, but they diminish consider- 

 ably in size as they approach the gastral surface. Whether the 

 tubes taper gradually or have a fusiform shape, can not be as- 

 serted. They are rather longer and more slender than in R. 

 Oweni^ and I think are not fusiform. The character of the inner 

 or gastrally directed surface of the outer wall is not presented 

 by the Lower Helderberg specimen. 



The Gastral Wall. 

 • Plate II, figures 4 and 6; Plate III, figures 3 and 5. 

 The points where the connecting tubes meet the inner wall 

 are united longitudinally by low, rounded elevations which a 

 fractured surface shows to be hollow. The channels thus formed 

 probably connect with the interiors of the tubes. The characters 

 of this surface are obscured by the nature of the pyrite which is 

 distinctly granular. The inner wall has a vesicular structure, 

 composed of labyrinthine canals.' Each tube terminates immedi- 

 ately beneath the center of one of the gastral rhombic pits, into 

 which, however, it does not open directly, but into the canal net- 

 work with which the gastral pits connect. Upon the gastral 

 side the rhombic depressions are traversed by horizontal parti- 

 tions, situated a little below the surface. These are thin and 

 continuous, and undoubtedly represent some .real feature in the 

 original organization. It is probably owing to these structures 

 that the radial tubes and the canals of the gastral wall are not 

 filled with the shaly matrix, but with a white crystalline mineral 

 impossible to confuse with it. 



