TETRAGRAPTUS. 61 



Fig. S6.-Tdragrapius Amii. ^llG tlieC^B Vaiy ill sizC aiul liumber aCCOvdillg to 



^^P ^ ^^ tlieir position on the main stipes. When completely 



f"' "' ' '' "— ' ■- developed they have an average length of about 



o mm., their walls are somewhat curved, and they 



^ V y Y V ^ ^1''-' approximately uniform in width throughout 



DibtiithoLL Lniu^cnRutofpart their length; the angle of inclination varies from 



ot I'l. V, fij^. i<.. al)out oO'' in the initial region of each thcca to 45° 



near the a})erture ; this is consequent upon the curvature of the thecal wall. 



Afjiulfics. — T. Amii forms a connecting link between Groups 1 and 4. In 

 general form and manner of growth it is closely related to T. qKadribmchiatiis,\mt 

 differs in the greater flexibility of the stipes, in their rapid widening, and in the 

 characters of the thecae. In these two latter characteristics it closely approximates 

 to T. serra, from which it differs, however, in general shape and mode of growth. 



RemarJci^. — This form was included by Hall in T. serra ( = T. hri/onoidcs (Hall), 

 ' Grapt. of Quebec Group,' p. 84), and though long regarded as distinct by 

 Lapworth no description has hitherto appeared. It was first recognised in 

 specimens from Canada, and is named after the palaeontologist Dr. Henri Ami, of 

 the Canadian Geological Survey. 



Horizon and Localities. — Arenig, Middle Skiddaw Slates. 



Lalce District : Randal Crag, Skiddaw ; White House Fell, Skiddaw ; E. 

 Dodd Wood. ^S'. Wales : Whitesand Bay. N. Wales : Aberdaron, 



Associates, etc. — T. Aviii occurs in the Skiddaw Slates of the north of England 

 associated with Tetrarj. serra and Bidijiruxj. extensns ; it is also found in Wales. 

 The best specimens known to us are in the Woodwardian Museum. 



Dependent Series. 



Tetragrapti with two pairs of main stipes diverging downward, but which all 

 tend to become approximately parallel. 



Group II. — Type T. frnticosiis. 



Tetragrapti in which the stipes are directed subvertically downward but 

 diverge at their distal extremities, and increase in width throughout their length ; 

 the tlieca) ai-e inclined at a moderate angle, and are in contact for the greater part 

 of their extent. 



Tetragraptus fruticosus (Hall). Plate VI, figs. 2^/, Ij. 



1858. GnqifiAUhu:^frulicosux, Hall, Gool. Survey Canada Hep., 18.57, p. 128. 

 1865. GraptoUthiix fruticosus, Hall, Grapt. of Quebec Group, p. 90, pi. vi, li^'s. 1 



