194 II. s. WILLIAMS — THE CI BOIDES ZONE AND [TS FAUNA. 



median fold and sinus which arc less than in the prevailing type of the 

 European euboides (plate 13). Throughout the Devonian of Europe and 

 Asia this species is found associated with a particular fauna at the base of the 

 upper Devonian, and its presence is regarded as indicative of a common 



•logic horizon; and since it can be traced regularly from country to 

 country, the terranes holding this fauna for the eastern continent may lie 

 regarded as approximately contemporaneous. 



There is a mutation of the sa species, called procuboides (figure L3, 



plate 13) by Kayser, occurring a little lower in some of the sections, the 

 distinctive features of which are seen to be characteristic of immature forms 

 of the true Rhynchonella euboides. 



Among the European forms there is considerable variation in the number 

 of plications on the median sinus; some specimens of the American type, 

 however, have as many plications as some of those of the European forms 

 not possessing the maximum number for that type. It is observed, further, 

 that these plications increase in number with the growth of the individual. 



This Rhynchonella venustula shows a closer affinity to the early mutation 

 of the European form in the characters common to both, but in its own 

 peculiar characters it shows a stage of development akin rather to the typical 

 euboides of Europe. To explain this fact we are led to believe that the two 

 forms had a common origin up to near the beginning of the Ouboides -tage, 

 but at that stage were separate and developed their local characteristics. 



All three species thus agree in bearing intrinsic evidence of a relationship 

 between the faunas of the New York and European Devonian, more intimate 

 during the stages from the Ouboides zone upward than for those previous to 

 that zutie. 



Comparison of European Species with American Forms. 



There are beside these a number of species belonging to the Ouboides 



fauna which do nol appear in the Tully limestone. Kayser has given a 



list of species typical of the Ouboides zone of Ais la Chapel le and the Eifel sec- 

 tion at Budesheim(p. 185). Gosselet has given lists for the Fraxnien (p. l Sv 

 There are li<t- given for the Merger Kalk, the Devonian limestone of 

 .--.lit 1 ■ Devonshire, and the various sections of Russia and the I Irals, l>v other 

 authors. In the Btudy of these lists the brachiopods also besl Berve our 

 purposes on accouul of the much fuller details we possess of their specific and 

 varietal character- and of their distribution. Among the species of these 

 lists the following are quite generally present in the Ouboides zone. 



Of Spirifers there are generally] recognized as belonging to the fauna 

 Spirifer nudus and S. euryglosus or pachyrynchiu. In the American Devon- 

 ian the tii-t representative of this race of Spirifer occurs above the Tully 



