MONOGRAPTUS. 417 



obverse) the thecal apertures more or less face the observer; while from the other 

 side (the reverse) no aperture, or iu some cases no apertural termination, is 

 visible at all. The result of this torsion, after compression, is the production of 

 very different "appearances" which appearances seem to have been often mis- 

 understood, and by some observers have even been regarded as of specific value. 



Many of the difficulties encountered in the endeavour to interpret correctly 

 the different appearances presented by the polypary in Groups IV and V of the 

 Monograpti will be obviated if it be borne in mind that the entire polypary in Mono- 

 graptus may theoretically be regarded as the left-hand vertical half of a polypary 

 of Diplograptus presented in its typical or obverse aspect, the right-hand half 

 being absent or suppressed. 



Consistently with this nomenclature, that special view of the outer surface of 

 the polypary in Mo?iograptus,m which the axis of the polypary lies towards the right 

 hand of the observer and the apertural margins of the thecaa towards the left, may 

 be regarded as the obverse aspect, and that in which these conditions are reversed 

 as the reverse aspect. 



Many of the more striking features available for specific distinction in the 

 aspect were those which were dependent upon the varying extent to which the 

 individual theca? in the course of their growth had become curved by flexure or 

 bending, and simultaneously turned sideways by torsion. The ultimate amount of 

 bending that each thecal axis underwent, measured in degrees from the line of the 

 axis of the polypary, constituted naturally its original angle of inclination or thecal 

 angle ; and the ultimate amount of twisting sideways, when measured in degrees, 

 constituted its original torsional angle. 



During and subsequent to fossilisation, however, the varying accidents of the mode 

 and degree of preservation, the amount and direction of compression and flattening, 

 the occasional splitting of the fossil and its containing rock lamina? during extrac- 

 tion, etc., have each brought about a definite effect, either singly or in combination. 



The ultimate result of all the foregoing is shown in that sum of appearances 

 which is exhibited in the actual view of the fossil as now seen by the palaeontolo- 

 gist and carefully represented upon his figures. The polypary may itself lie 

 normally in the left-hand (obverse) or right-hand (reverse) view, but the 

 correct interpretation of all the detailed appearances presented is often a matter 

 of extreme difficulty. 



The graptolites included in the present group may be further divided into sections 

 and sub-sections according as to whether the theca? are uniform or bi-form, and 

 according to the kind of curvature of the polypary. It is obvious, however, that 

 a classification on such a basis is largely a matter of convenience. 



A. "With uniform thecae — 



1. With approximately straight polypary. Type Monograptus priodon. 

 M. pandus, M. Marri, M. cultellus, M. riccartonensis, M. Flemingii, 



