HYDROZOA. GRAPTOLITOIDEA 65 



the sicula {e.g. Didymograptus) ; if they were fixed to 

 floating sea-weeds their wide geographical distribution 

 would be readily accounted for. Ruedemann has described 

 specimens of Diplograptus pristis (fig. 19), from the 

 Utica Slate (Ordovician) of New York, which consist of 

 a number of individuals radiating from a centre where 

 they unite by the distal prolongations of their virgulas ; 

 at the point of union there is a small, nearly square, 

 chitinous sheath which is similar in appearance to the 

 central disc of Tetragraptns ; below this is a larger 

 quadrate body, apparently vesicular, which was at first 

 regarded as a float or pneumatocyst, but later as probably 

 an organ of fixation at the base of the colony. Around 

 the small disc are from four to eight globular vesicles, 

 which Ruedemann considers to be gonangia, since they 

 contain siculse; the siculae sometimes pass out and develop 

 into fresh colonies, but in other cases they remain attached 

 to the parent, and, by the growth of the virgula, extend 

 outwards, and subsequently hydrothecse arise in the usual 

 way. In some other species of Diplograptus {D. vesicidosus, 

 etc.) a single vesicle is sometimes found attached to the 

 distal end of the virgula. 



The genera of graptolites at present accepted are 

 based, to a large extent, on the number of branches of 

 the polypary ; but Nicholson and Marr consider that this 

 feature is of less importance than was formerly supposed, 

 and that a classification which shows the genealogical 

 relationships of the forms should be founded chiefly on 

 the characters of the hydrothecse and, to some extent, on 

 the angle of divergence of the branches. The early grapto- 

 lites, such as Bryograptus, appear, at first sight, to be 

 more advanced than the later types {e.g. Monograptus), 

 on account of their more complex branching ; but in the 



w. p. 5 



