DIPLOGRAPTUS. 219 



appearances after compression, the simplest types are seen in Petalograptns, in 

 which the edge was even and devoid of ornament; so that the denticle (Nich.), 

 — i.e. the apparent corner or angle constituted in compressed forms by the 

 meeting of the apparent profile of the apertural margin and the apparent 

 profile of the free edge of the theca — becomes almost rectangular. In Ghjpto- 

 graptus the outline of the apertural edge was somewhat undulate, so that the 

 denticle becomes either sharp and inclined {cuspate) (Fig. 171b), or rounded and 

 re-curved (rostrate) (Fig. 171 d), according to the direction of compression. In the 

 sub-genus Orthograptus the undulation of the apertural margin Avas in many forms 

 so pronounced as to give origin to two well-marked elevations or lobes lying 

 between three depressions, so that the aperture somewhat resembled in shape the 

 lip of a jug. As the apertural edge itself often had a distinct thickening or 

 flange we are presented, in compressed specimens belonging to this sub-genus, 

 with a great variety of appearances, due, some to the original distinctions in the 

 forms of the polypary and thecse, and some to the different directions in which the 

 compression has been effected. The apparent profile of the apertural margin of 

 the theca may appear straight, undulate, lobate, notched or broadly-rounded in 

 outline ; horizontal, inclined or everted in direction ; and the denticle cuspate, 

 rostrate or mucronate in shape. In the extreme forms of the sub-genus the 

 apertural lobes are prolonged into distinct spines, sometimes of great length and 

 stiffness. 



The views in which the compressed polyparies and thecx, are presented are 

 varied. In all the flattened examples of species belonging to the genus Diplograptus 

 the commonest view shown is the profile (or, better, bi-profile) view (Fig. 145/), in 

 which the septal groove, if present, runs down the middle of the symmetrically 

 compressed polypary. In this view, in the majority of cases, it is impossible to 

 determine whether the obverse or the reverse aspect is presented. The scalariform 

 (or true ventral) view so commonly found in Glimacograptus is exceedingly rare. 

 In the sub-genus Petalograptus this bi- profile view is almost the only one met 

 with. In Glgptograptus the bi-profile view (here with " excavations ") is charac- 

 teristic (Fig. 171b) ; but a common one is the sub-scalarifonn, quarter-face, or 

 oblique view (Fig. 167 a), in which the apertural margin shows for three quarters 

 of its width on one side of the polypary, and for one quarter on the other. In 

 Mesograptus, as a general rule, the views vary in different parts of the same poly- 

 pary, and in the sub-group Amplexog raptus we find two bi-profile views — an obverse 

 aspect (Fig. 184 c), in which the polypary and thecae resemble those of Climacograjt- 

 tits, and a reverse aspect (Fig. 184t?), in which they resemble those of Orthograptus 

 or Glgptograptus. In the sub-genus Orthograptus the bi-profile view (Fig. 154 b) is 

 the dominant one, but the most characteristic is the quarter-face or sub-scalariform 

 view (Fig. 161 d), in which, as in 0. vulgatus, the denticles often show as cuspate 

 on one of the lateral edges and rostrate on the other, or, as in 0. quadrhmicronatus, 



