GRAPTOLITOIDEA. 



217 



Fig. 101. — Tetragraptus quadribrachl- 

 atus (after Hall). Ordovician (Skiddaw 

 and Quebec groups). 



angle ("sicular angle ") being necessarily over 180 . The species 

 of Didymograptus are confined to the Ordovician rocks. Closely 

 allied to the preceding is the genus Tetragraptus (fig. 10 1) in 

 which the polypary consists of four simple branches springing 

 from a central, non-polypiferous connecting-process or "funicle," 

 which bifurcates at each end. The 

 base is sometimes provided with 

 a peculiar corneous disc, similar 

 to that which characterises various 

 species of Dichograptus. The 

 species of Tetragraptus are all 

 Ordovician (Arenig). In the genus 

 Dichograptus (fig. 102), again, the 

 polypary consists of eight simple 

 branches, which arise from the 

 same number of divisions of a 

 non-polypiferous basal process or 

 "funicle." In many cases, the di- 

 visions of the " funicle " are envel- 

 oped in a species of horny " disc " 

 or plate (fig. 102), which is be- 

 lieved to have been composed of 

 two laminae. The functions of this disc are doubtful, but it has 

 been compared with the "float" of the Portuguese-Man-of-war 

 {Physalia) or of Velella among the recent Oceanic Hydrozoa. The 

 species of Dichograptus are confined to the lower portion of the 

 Ordovician series (Arenig); but allied genera, in which the polypary 

 is irregularly branched, are found in the Upper Cambrian deposits, 

 and are represented in the Ordovician rocks. 



The only remaining family of the Monoprionidian Graptolites is 

 that of the Dicranograptidce, comprising the two genera Dicrano- 

 graptus and Dice/iograpttis, in both of which the polypary is com- 

 posed of two branches, and the terminations of the hydrothecse are 

 isolated and characteristically incurved, so as to bring the cell- 

 apertures within the line of margin of the branch (fig. 103). In 

 the genus Dicellograptus (fig. 103, b) the two branches of the poly- 

 pary are wholly free, and diverge from a basal "sicula," in such 

 a manner as to resemble the genus Didymograptus. The angle 

 included between the polypiferous sides of the branches (" angle of 

 divergence") is, however, greater than 180 , while the opposite 

 angle ("sicular angle") is necessarily less than 180 . In the cu- 

 rious genus Dicranograptus (fig. 103, a) we have a transition be- 

 tween the Monoprionidian and Diprionidian groups of Graptolites, 

 the two branches composing the polypary being adherent by their 

 dorsal surfaces for a longer or shorter distance, and then diverging 



