LowEE Heldeeberg Fauna. 287 



ure 3. The summit plates are structureless and the tangential 

 rays have not been observed in this specimen. The pillars are 

 connected directly with the summit plates, and like them are of 

 crystalline calcite. There is no trace of a central canal. A. little 

 below the summit plate the pillars expand suddenly and then 

 taper slightly. They are marked by distinct annular constric- 

 tions or lines of growth. At their ianer extremities they meet 

 the gastral wall, or expand to form it. At any rate, the wall is 

 continuous, and the demarkation of the plates no longer exists. 

 Opposite the pillars, and occupying nearly the whole of each 

 plate on its outer side, is a rounded elevation covered by anasto- 

 mosing channels, as described above. This structure seems to 

 be characteristic of the gastral surface. As the inner wall of 

 this specimen exhibits neither the canals nor perforations described 

 by HiNDE and Billings, it confirms the observations of Kaeff^ 

 who considers these features to be due to preservation. Sections 

 illustrating the various structures of the specimen are shown on 

 Plate YI, figures 1, 2, 3, 4. 



jReceptacuUtes JV^eptuni. 



In the American Museum of Natural History, IN^ew York city, 

 there is a siliceous representative of 12. Nejptuni (the type species), 

 which in the structure of its outer wall agrees with the pyritized 

 specimen from the Helderberg. The tubes are hollow, their 

 wall continuous with the radiating ridges and their central canals 

 open into the rhombic pits, which are not occupied by summit 

 plates 



The foregoing comparison illustrates the conflicting nature of 

 the evidence afforded by Receptaculites. It shows that speci- 

 mens apparently well preserved contradict others of the same 

 genus, and that structures were maintained or obliterated as the 

 result of slight charges in the conditions of preservation. The 

 specimen of R. infiondibuliformis is certainly more trustworthy 

 than any other example of the genus before noticed, and affords 

 a more accurate notion of the original structure and its details. 

 Still the belief is unwarranted that this specimen represents the 

 organism in its entirety, and that nothing further remains to be 

 studied. Enough, however, has been preserved to direct aright 

 the consideration of the affinities and systematic position of the 

 genus. 



