PK0F o E. W. CLATPOLE ON HELICOPOKA. 37 



The discovery of other spiral Fenestellids is rendered more pro- 

 bable, indeed almost certain, by the early date of the first species 

 here described. This form has been hitherto sharply limited to 

 one horizon, that of the Keokuk, Warsaw, and Kaskaskia lime- 

 stones in the Lower Carboniferous. The occurrence of H. lati- 

 spiralis in Silurian rocks gives us the prospect of finding iu the 

 intervening Devonian beds, which in N. America have not yet been 

 well searched, species additional to the single one described above 

 from the Corniferous Limestone. 



It may serve to place the relations of the groups of fossils to which 

 allusion has been made in a clearer light if they are arranged in a 

 tabular form, thus : — 



Pores arranged in two rows. 

 Polyzoary not fenestrate: 



Polyzoary irregularly branched Galuconome. 



Polyzoary plumose Ichthyorhachis. 



Polyzoary fenestrate. 

 Polyzoary spiral : 



With a straight central axis Archimedes. 



Without a straight central axis, Helicopora. 



Polyzoary not spiral : 



Polyzoary with strong midrib Ptilopora. 



Polyzoary with strong lateral ribs Lycopora. 



Polyzoary without ribs , Fenestella (proper). 



It will be seen that the only ground of subdivision among the 

 fenestrate group is the mode of growth. By referring to the 

 definition given at the beginning of the paper the details of this 

 structure may be seen. 



Prof. Hall writes (Proc. Amer. Assoc. Adv. Sci. 1856, p. 177) : — 

 " In all the essential characteristics the foliate expansions of Archi- 

 medes correspond to Fenestella, according to the extended description 

 of this genus by Mr. Lonsdale ; and in detached fragments cannot 

 be distinguished generically from other forms of the same genus." 

 And further, in the ' Palaeontology of Iowa ' (p. 651) : — " Their 

 mode of growth, however, is quite distinct ; the flabelliform expan- 

 sion acquiring a solid central axis, around which it revolves in an 

 ascending spiral form, spreading equally on every side." 



This is equally true of the two coordinate groups (whether genera 

 or subgenera matters little) Ptilopora and Lycojwra, and in a less 

 degree of Fenestella (pr.), inasmuch as it is characterized by negative 

 features, if the expression may be allowed. The mode of growth, 

 then, in this family is evidently admitted as a principle of sub- 

 division, and it would be difficult or impossible to obtain any other. 

 No Penestellid of spiral growth has yet been described without the 

 axis of Archimedes, and no Archimedes without this strong, straight 

 central shaft, and almost always stony braces between the whorls. 

 Consequently Helicopora takes up ground hitherto unoccupied, filling 

 a gap and supplying a connecting link between Fenestella and 

 Archimedes. 



It is, in conclusion, worthy of remark that the Old World has 

 not contributed any spiral Polyzoan to the catalogue, unless it be 



