DIPLOGRAPTUS. 217 



Genus DIPLOGRAPTUS, M'Coy. 



1850. Diplograpsis, M'Coy, Ami. Mag. Nat. Hist, [5], vol. vi, p. 270. 

 1854. Diplograpsus, M'Coy, Brit. Pal. Foss., p. 3. 



Polypary bilaterally symmetrical ; rectangular, sub-cylindrical, concavo-convex, 

 or tabular in section ; biserial throughout ; test continuous. 



Thecse sub-prismatic or sub-cylindrical tubes, ventral walls typically inclined 

 and more or less straight ; apertural margins even or undulated. 



The polypary in Diplograptus varies much in size, ranging from a few milli- 

 metres to many centimetres in length ; it may be narrow or broad, when broad 

 attaining its maximum width either gradually or within a short distance of the 

 proximal end. 



The sicula as a general rule is visible only in the obverse aspect of the polypary, 

 and its apex is more or less embraced and completely concealed by the bases of the 

 early thecse. 



The polypary in Diplograptus presents greater difficulties of description and 

 interpretation, perhaps, than that in any other genus. These are due not only to 

 the differences in the outline and transverse section of the original and uncom- 

 pressed polypary in the different species, but also to the diversity in the original 

 form of their thecse ; and these differences are complicated by the fact that 

 British examples are usually compressed to mere films. 



So far as may be inferred from such specimens of Diplograptus as have hitherto 

 been met with preserved in relief or sub-relief, the original distinctions among the 

 various groups and species were those connected with (1) the general outline of the 

 polypary, and (2) its transverse section ; (3) the general form of the theca, (4) the 

 shape of its apertural margin, and (5) its ornaments. These primary distinctions 

 may all undergo accidental modifications during the process of compression in the 

 rock, giving origin to secondary appearances, which vary with the direction and 

 the amount of compression. 



The general outline of the polypary , in the majority of forms belonging to Diplo- 

 graptus, resembles that in Gliniacograptus, being as a rule parallel-sided, or taper- 

 ing towards the proximal extremity ; but there are almost all gradations, from 

 parallel-sided to fusiform, foliate, and even wedge-shaped. 



The polypary appears to have been in no case quite cylindrical as in Climaco- 

 graptus, but was somewhat concavo-convex, the concave side being narrower than 

 the convex. In the majority of forms, however, the departure from symmetry was 

 apparently but slight. 



As respects the distinctions in the form of the transverse section of the polypary, 

 there is a first group of species— of which Diplograptus quadrinvucronatus, Hall, 



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