ARCH^EOCYATHINiE. 



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V 



y«i. 



The skeleton in Spirocyathus atlanticus is conical in form, with a 

 general resemblance to a simple coral, its interior being occupied by 

 a longitudinal tubular cavity (fig. 72 bis, c and d). The space 

 between the inner and outer walls is occupied by reticulated calcar- 

 eous tissue, in which a radial structure is hardly recognisable, while 

 distinct vertical " septa " do not appear to be present. The spaces 

 or irregular canals formed 

 by this calcareous reticu- 

 lation open on both the >ctT?l 

 outer and inner surfaces 

 of the skeleton by round- 

 ed or oval pores. In 

 this case, also, the skele- 

 ton seems to have been 

 originally calcareous, and 

 there is no evidence of 

 any spicular structure. 

 Spirocyathus atlanticus 

 occurs in the Cambrian 

 rocks of Canada and the 

 United States, and ap- 

 parently allied types have 

 been discovered by Bor- 

 nemann in strata of the 

 same age in Sardinia, 

 and have been described 

 by him under the name 

 of Protopharetra. 



The Archceocyathince 

 are among the most 

 ancient of all known 

 fossils, and are therefore 

 of peculiar zoological interest. It is not, however, possible at 

 present to come to any absolutely certain conclusion as to their 

 systematic position. As their skeleton seems to have been un- 

 doubtedly originally calcareous, and as there is no certain evi- 

 dence (now that the form described by Billings as Archcsocyathus 

 Minganensis is known to be a Sponge) that their structure was 

 spicular, it would not be possible to refer the group to the Pori- 

 fera. On the other hand, as pointed out by Hinde, the Archceo- 

 cyathince show certain unquestionable points of relationship to 

 the Madreporarian Corals. This is particularly seen in the reticu- 

 lated structure and usually more or less radiate arrangement of 

 the skeletal framework, the skeleton being in many respects com- 

 parable with that of such a Perforate Coral as Calostylis. At the 







m 



mi 



Fig. 72 bis.— a, A specimen of EtJunophyllum Rensse- 

 Icericum, Ford, of the natural size; b, Transverse section 

 of the_ same, showing the outer and inner walls and the 

 radiating septa (s) ; c and d, Transverse and longitudinal 

 sections of Spirocyathus atlanticus, Bill. sp. p, Pores in 

 the outer wall ; p', Pores in the inner wall ; c The in- 

 ternal cup or chamber. From the Cambrian rocks of 

 North America. (After Walcott.) 



