68 CARBONIFEROUS CEPHALOPODA OF IRELAND. 



(PI. XX, fig. 3), the keels haviug become obsolete or very faint. The aperture 

 has a deep hypouomic sinus, on eacli side of which there are prominent lobes or 

 crests (PI. XX, figs. 3 and 6) ; viewed laterally it presents a graceful sigmoid 

 curve (PI. XX, fig. 8). 



The ornaments of the test in the young shell consist of five irregularly spaced, 

 exceedingly fine, thread-like longitudinal lines on each side of the peripheral area ; 

 these are re[)laced by two prominent keels in the adult shell, as already described. 

 The whole surface is covered with very fine, transverse lines of growth, which 

 form in the young shell minute, but quite distinct, crenulations in crossing the 

 peripheral keel. In the adult shell they form a deep sinus upon the peripheral 

 area, corresponding with that of the aperture. Upon the peripheral area of the 

 adult shell a series of extremely fine longitudinal lines cover the two keels, but 

 they do not extend beyond them laterally. 



There appears to be no impressed zone in the young shell, but instead a slight 

 flattening of the dorsal area is seen upon the inner whorl. The dorsal area of 

 the initial volution (bordering the central vacuity) is raised into a faint median 

 keel (PI. XX, fig. 9), with other still fainter ridges on each side of it. 



Upon the cast of the body-chamber in the adult shell the " Runzelschicht " is 

 very distinctly preserved, consisting of rows of fine circular pittings arranged 

 transversely to the longer axis of the shell. 



The septa are extremely numerous, six occupying the space of 13 mm. near the 

 body-chamber of an adult shell whose lateral diameter is from 11 mm. to 13 mm. 

 at the place measured (PL XX, fig. 2). Upon the peripheral area of a young shell 

 five occupy the space of 8 mm. ; they here curve slightly backwards, as also upon 

 the sides, making a sharp bend in crossing the angular margin (PL XX, fig. 7h). 



The siphuncle is situated in the upper third of the septum, maintaining this 

 position apparently in all stages of growth (PL XX, figs. 4 and 9). 



Dimensions. 



Diameter of shell 



,, umbilicus (from edge to edge) . 



,, ,, (from suture to suture) 



,, central vacuity 



Height of outer whorl (dorso-ventral) 

 Thickness of whorl at umbilical edge 



Affinities. — This species is most nearly related to G. planotergatus, from 

 which it is distinguished by its slower rate of tapering, the strong keels upon 

 the peripheral area, and the incipient folds upon the inner whorls. The width 



Adult. 



Young. 



57 mm. 



32 mm. 



32 „ 



21 „ 



20 „ 



15 „ 



? 



7 „ 



19 „ 



11 „ 



28 „ 



14 „ 



