428 DAVIDSON ON FOSSIL BRACHIOPODA. CHAP. xxii. 



known as Lejjtoena depressa, which we must now call, in obe- 

 dience to the law of priority of nomenclature, Anomites (or 

 JStrophomena) rhomboidalis, Wahlenberg. No less than fifteen 

 commonly received species are demonstrated by Mr. David- 

 son, b}^ the aid of a long series of transitional forms, to apper- 

 tain to this one type; and it is acknow^ledged by some of the 

 best w^riters that they were induced to give distinct names 

 to some of the varieties now suppressed on purely theoretical 

 gi'ounds: namely, because they found them in rocks so widely 

 remote in time, that they deemed it contrary to analogy to 

 suppose that the same species could have endured so long, — 

 a mode of reasoning analogous to that which leads some 

 zoologists and botanists to distinguish by specific names 

 slight varieties of living plants and animals met with in very 

 remote countries, as in Europe and Australia, for example, — it 

 being assumed that each species has had a single birthplace 

 or area of creation, and that they could not by migration 

 have gone from the northern to the southern hemisphere 

 across the intervening tropics. 



Examples are also given by Mr. Davidson of species which 

 pass from the Devonian into the Carboniferous, and from that 

 again into the Permian rocks. The vast longevity of such 

 specific forms has not been generally recognized in conse- 

 quence of the change of names which they have undergone 

 ■when derived from such distant formations, — as when Atrijpa 

 unguicularis assumes, when derived from a carboniferous 

 rock, the name of Spirifer Urii, besides several other syno- 

 nyms, and then, when it reaches the Permian period, takes 

 the name of Spirifer Clannyana (King) ; all of which forms 

 the author of the monograph now under consideration asserts 

 to be one and the same. 



No geologist will deny that the distance of time which 

 separates some of the eras above alluded to, or the dates of 

 the earliest and latest appearances of some of the fossils 



1 



