LowEE Helderbeeg Fauna. 285 



rhombic areas by two sets of intersecting ridges. Shallow 

 rounded canals transverse each rhombus and connect the 

 opposite angles. Of the four, that which runs between the 

 center and one of the obtuse angles is noticeably on a higher 

 plane than the others, the opposite one being lower than the 

 rest. Nothinof similar to this arrano^ement has been observed in 

 It. infundibuliformis. From the center of each rhombus a 

 radial canal perforates the specimen, opening on the gastral side. 

 Just below the surface the canals suddenly expand, then taper 

 gradually, until they meet the inner surface. The diameter 

 where they terminate is considerably larger than that at the 

 starting point. The canals are not terete, but are modified by 

 annular constrictions and dilatations like the growth-lines of a 

 coral. The inner surface, also, is marked by ridges, which, 

 however, do not make a regular rhombic pattern, but wind 

 irregularly among the tubular orifices. Sometimes the openings 

 of the tubes are surrounded by shallow, circular depressions ; 

 sometimes there is an appearance as if canals ran from the tube 

 to the acute angles of the rhombus ; and sometimes the inter- 

 sections of the ridges are emphasized by monticules. 



Billings calls attention to the fact that in magnesian lime- 

 stones, where the hard parts of fossils are in general totally 

 removed, so that the cavities once occupied by them remain 

 empty, Keceptacdlites is found in the same condition (p. 386 loo. 

 cit). This is true of R. Oweni, which bears every evidence of 

 being a cast. In most specimens, then, the remains consist of the 

 filling of the intermural space, with casts of the outer surface of 

 the inner wall, the inner surface of the outer wall and of the con- 

 necting tubes. 



The structures in It. Oweni and the pyritized specimen of R. infun- 

 dibuliformis correspond very closely, although the former, as a 

 cast, represents inrer surfaces of the skeleton and the latter 

 seems to preserve the outer surface. Details in R. Oweni are 

 imperfect in character, and partly obliterated by the granular 

 nature of the matrix. 



Another feature, the configuration of the external surface, has 

 been noticed. by Prof. Hall.*. The specimen representing it is 

 preserved in the American Museum of Natural History, New 



♦ Hall, 1S61. Rep. Prog. Geol. Surv. Wisconsin, p. 15. 



