36 PKOF. E. W. CLA.YPOLE ON HELICOPORA. 



as truly referred to the genus Archimedes. Nor would the impres- 

 sion be unwarranted ; but a comparison of the fossil itself with an 

 Archimedes brings out the distinctions more clearly than they can be 

 shown in a figure. Moreover, since the spiral form of polyzoary 

 is found outside of the genus Archimedes, it can no longer be held as 

 an essential characteristic of that genus. Archimedes must there- 

 fore be characterized by the spiral polyzoary in connexion with the 

 axis ; and in all the hitherto known species of that genus this axis is 

 strong and straight, forming a true central line. Consequently the 

 distinction between Archimedes and Helicopora archimediformis is 

 easily seen. 



Were the difference, however, less obvious, the reference to Heli- 

 copora would not necessarily be incorrect. As palaeontology ad- 

 vances, we continually find new forms filling the gaps previously 

 existing between others already known ; and such connecting links 

 may in many cases be referred with justice to either of the two 

 genera which they connect. Such cases are constantly occurring, 

 and must be expected to occur yet more frequently in the future *. 

 In the present instance it is evident that the spiral mode of growth 

 was not uncommon among the older Fenestellids, and can not there- 

 fore be regarded as a mere " sport." Its range at present is from 

 the Upper Silurian to the Lower Carboniferous. The large size of 

 H. latispiralis forbids our supposing that it was the first of the kind, 

 and it would be extremely rash to assume that the Kaskaskia species 

 were the last. We may reasonably anticipate the discovery of other 

 forms which will connect by closer links those already known, and 

 perhaps establish a series from beds older than the Silurian to beds 

 newer than the Carboniferous. 



On the larger question whether or not all these groups should be 

 thrown together again as Fenestella, this is not the place to enter : 

 the numerous distinct forms found in the North- American Palaeozoic 

 rocks would make such a discussion both difficult and unsatisfactory 

 in the present state of our knowledge. There are more than sixty 

 species already described under the generic or subgeneric names 

 quoted above ; and to whatever extent these may be hereafter 

 reduced, enough would still remain to render the retention of the 

 present term very advantageous. 



* In this connexion the following extract from Mr. G-. W. Shrubsole's 

 recent paper on the " British Upper Silurian Fenestellidge," will be valuable as 

 confirmatory evidence : — 



"As to the question whether Fenestella intermedia, with its three rows of 

 pores, ought also to be included with Polypora, it may fairly be left open for con- 

 sideration. ... It may be, and is, difficult in practice to draw the line as to where 

 Fenestella ends and. Polypora begins. The genus Polypora was founded by 

 Prof. M'Coy for that division of the Fenestella family having more than two 

 rows of cells on the interstice ; the usual number of rows of cells in Polypora 

 is from three to ten. These intermediate or compound forms, as Polypora 

 incepta or Fenestella intermedia, were then unknown, and the difficulty as to 

 classification had not arisen. Fenestella intermedia is clearly one of those 

 connecting links between allied, genera which, while they serve to unite the 

 family as a group, are somewhat difficult to classify." — Q. J. Gl. S. vol. xxxvi. 

 p. 251. 



