290 



PALEONTOLOGY 



thecae on the opposite side to the sicula. These thecae 

 lie obliquely and overlap one another for about half their 

 length ; the free portion is bent first outwards and then 

 downwards, so as to have a hook-like appearance. The 

 curvature of the theca and the direction of the aperture 

 are thus the opposite to those of Climacograptus. The 

 first theca grows upwards from the sicula : the first stage 

 of downward growth, corresponding to the permanent 

 direction of Didymograptus, and still preserved in Climaco- 

 graptus, is skipped altogether mMonograptus. 



a b c 



FlG. 87. MONOGRAPTUS PRIODON (BRONN), WENLOCK SHALE. 



a, Initial part of polypary, showing sicula, with virgula arising from its 

 apex ; b, distal thecae, reverse aspect, apertures partly embedded in 

 rock; c, distal thecse in low-relief, obverse aspect, showing apertural 

 margins. ( X I ) (After Elles and Wood.) 



It is very probable that Monograptus was not a floating 

 organism, but was fixed to the sea-bottom. 



The following classification of the graptolites is that 

 given in the monograph by Elles, Wood, and Lapworth. 

 Professor Freeh of Breslau unites the three first families 

 as Axonolipa (without a virgula), and the rest as 

 Axonophora (with virgula). 



Family DICHOGRAPTID^E. Uniserial Graptoloidea, with 

 bilateral polyparies, bearing simple subcylindrical thecae. 



