GRAPTOLITES OF NEW YORK, PART 1 503 



as it can be reconstructed for the Lower Siluric age from the study of the 

 littoral faunas.^ One of these is the appearance of the peculiar generic type 

 Groniograptus with the same species, G. t h u r e a u i, m Victoria, Australia, 

 in the south Australian sea of that age which formed presumably a south- 

 western embay ment of the Pacific- American basin, and in the homotaxial 

 Tetragraptus beds of Point Levis, Canada, and the Deep kill, New York, 

 while no trace of it has been found in the corresponding European beds. 

 The only explanation of this abnormal distribution can, m the writer's 

 opinion, be found in the assumption that the supposed " Levis channel " of 

 Ulrich and Schuchert had at that time not only an open connection with the 

 northwestern (St Lawrence) embayment of the North Atlantic basin, but also 

 at its southern terminus with the Pacific- American basin, and that oceanic 

 currents connected the habitats of Goniograptus thureaui, at 

 present antipodal regions. [^See chart] 



Another irregularity of distribution can be found in the appearance of 

 the Dendroidea, with the genera Dendrograptus, Dictyonema, Callograptus 

 and Ptilograptus, in great force at certain localities, while they are absent 

 in homotaxial beds at others. Thus Lap worth and Hopkinson [1875, p.635] 

 enumerate from the graptolite beds on Ramsey island, Wales, 10 Dendroidea 

 to only six Graptoloidea. From the whole St Davids district they record 

 15 Dendroidea to 20 Graptoloidea, while the Dendroidea are entirely absent 

 in the corresponding beds of the English Lake district, and are rare in 

 the homotaxial beds of the Quebec region. They are, further, extremely 

 prominent in species and individuals in the third Deep kill zone, that of 

 Diplograptus dentatus [see table of distribution of species, p.504], 

 while they are absent in the same zone at Point Levis and in the Lake district; 

 and are hardly represented at Mt Moreno. The peculiarly local distribution of 

 this class of graptolites would seem to suggest a mode of existence different 

 from that of the other members of the group [p.514]. 



^ See Freeh. Lethaea palaeozoica, v.l, chart 2. 



