182 BRITISH GRAPTOLITES. 



J 11 the Diplograptidse the protection of the sicula is more perfectly attained 

 than in any other family, since it is often completely imbedded in the proximal 

 part of the polypary. 



The forms belonging to the family are generally preserved as thin films, having 

 been compressed in such a way as to show the polypary and its two lateral series 

 of thecae all flattened down together to their common median plane. As a conse- 

 quence the diagnostic criteria primarily employed or implied in the description of 

 the various genera and species are those detailed appearances which are presented 

 under these special conditions in this normal aspect (profile view). In the progress 

 of research these conventional criteria become supplemented, interpreted, or 

 modified by information furnished by flattened examples presenting the ventral 

 aspect (scalariform view), or by specimens preserved in the round {in relief), etc. 



The best known genera of the family are Climacograptus and Diplograptus. 



In the genus Climacograptus the ventral wall of the theca in the profile view shows 

 sigmoid curvature, its free outer portion is approximately vertical, and the margins 

 of the aperture, which is situated within a well-defined "excavation," are horizontal 

 or but slightly inclined, though sometimes introverted, and occasionally introtorted. 



In the genus Dvplograptus the ventral wall of the theca in the profile view is 

 approximately straight or gradually curved throughout, and is distinctly inclined, 

 while the margins of the aperture are typically everted. 



Genus CLIMACOGRAPTUS, Hall. 



1865. Climacograptus, Hall, Grapt. of Quebec Group, p. 111. 



Polypary bilaterally symmetrical, biserial throughout. 



Thecae tubular, ventral walls with every degree of sigmoid curvature ; 

 apertural margins typically horizontal, situated within well-defined " excavations," 

 occasionally introverted and rarely introtorted. 



-ciinuicogmpius Schareniergi, The polypary in CUmacog mptus varies greatly. 



In some forms it is characteristically minute, while 

 in others it attains great dimensions. In some 

 species the polypary has sub-parallel margins and 

 attains its maximum breadth quickly, while in 

 others again it shows marked tapering. 



The sicula is never visible for its entire length in 

 adult specimens, being more or less completely 

 Typical species of the genus ciimaco- concealed by the bases of the earlier thee*. 



EfSttaTa KTefticTfcel The mode of development of the polypary in the 



' " n a3 



Lapv 



edge, the septal groove etc DoWs genus fe { \ nx{ characteristic of the Diplograptid 



Linn. Lower Ha.rtli.-ll Shales. C..11. " 1 fe I 



Lapwortn. [ n general. 



