THE FAUNA OF THE ST. JOHN GEOUP. 121 



mentary form thau iu the other Acadiau species of this group ; but it can be proved for 

 the others of which embryonic and larval stages are known ; for the retreat of the eyelobe 

 from the front of the shield proceeds ^an' /?assM with the axial contraction of the glabella. 



Among the Acadian Conocorypheans occur several species which may be considered 

 rej)resentative of Old World forms, as has already been remarked in the opening part of 

 this paper ; and it may be said that among the Paradoxides similar representation occurs. 

 This is very obvious on a comparison of P. Eteminicus with P. rugulosus (Corda) ; and P. 

 Acadicus, notwithstanding its diminutive size and differing eyelobe, may not inaptly be 

 placed beside Plutonia Sedgwickii ; in both species we have a granulated surface and deeply 

 cut glabellar grooves ; and in both may be traced peculiarities of form which ally the 

 shield to that of Anopolinus. 



It is a notable fact, however, that no Paradoxides with short eyelobes and no Auopo- 

 leni have been recovered from the Acadian strata, although the latter genus occurs in 

 Newfoundland and the large Paradoxides are found both there and in Massachusetts. It 

 may be remarked, however, that in both of these areas the above types wanting in the 

 Acadian region are associated with genera that range upward in the Cambrian formation ; 

 and the reason why they are not found at St. John is probably, that the known fossiliferous 

 belt in the Acadian area belongs to a lower or older horizon in the Cambrian formation, 

 than has yet been reached in these other Cambrian areas on the Atlantic coast.^ 



The antiquity of the Acadian fauna can best be appreciated when its forms are com- 

 pared with those of the British Cambrian rocks, for that is the nearest of the European 

 areas occupied by rocks of this age and the one which contains the largest number of sim- 

 ilar species. The great thickness of the "Welsh bed has distributed the genera of Cam- 

 brian age over a wide vertical range, and thus given a better opportunity of estimating 

 their chronological value. 



It has been the custom to speak of the fauna of the Acadiau horizon in the St. 

 John groiip as equivalent to the Menevian. But the Menevian, as now limited by Dr. Hicks, 

 does not include all the measures originally assigned to it. These have been found to con- 

 tain two faunas — an upper, which is now called Menevian, and a lower, that of the Solva 

 group. 



The more we know of the Acadian fauna, the less does the restricted Menevian seem 

 the horizon to which it should be assigned. The wonderful richness and variety of the 

 Menevian fauna tempts one to adopt this correlation ; but this very feature of the Old 

 World group should put us on our guard against carelessly associating with it an equally 

 rich assemblage of living forms, which, from their very abundance, are likely to contain 

 a number of representative species. 



In the Acadian fauna, as thirs far known, the great Paradoxides with short eyelobes 

 are wanting; so also are the genera Anopolinus, Agniulos (Arionellas) of the type A. ceticepha- 

 lus, Microdisais of the type M. putictalusj- Erinnys holocephalina, etc. If, on the other hand, 

 the fauna of the Acadian horizon be compared with the oldest British Cambrian fauna, 

 a strong resemblance between the species on the two sides of the Atlantic is at once ap- 



' Except the fossils of Manuel Brook, Newfoundland, which appear to be of the Acadian horizon. 

 ' A similar species occurs in the St. John group, but apparently at a, different horizon. 



Sec. IV., 1884. 16. 



