74 DEVONIAN FAUNA. 



G. divisus, Miinst.,^ is a discoidal and not ellipsoidal form ; its back is broader, 

 and of a more subquadrilateral shape ; it is marked with sulci ; and its lobes are 

 large and acute. 



G. sulcatus, Miinst./ has a large elongate lateral lobe of the same size as the 

 central lobe ; the saddles are much deeper, and the section more squared. His 

 other species have either larger umbilici or more lobes in the suture-line. 



Turning to Sandberger, we find an approach in G. amhlylohus,^ which is to be 

 distinguished by the presence of deep sulci and by a wide central saddle, and a 

 dorsally steep side lobe. All the other shells figured by him under the name 

 G. retrorsus^ are circular, not elliptic, and their lateral lobes are never steeper 

 ventrally than dorsally. 



F. A. Romer'^ figures a variety of G. retrorsus with closed umbilicus, which 

 differs from ours in not being elliptic, and in not having the sides of the aperture 

 arched forward. In a later part of his work^ he describes another example of 

 G. retrorsus typus, from the "Disjuncta" beds, which differs in much the same 

 particulars. 



G. atratus, Groldf., as figured by Beyrich,^ is a minute flat shell very similar to 

 fig. 12 in shape, and has a closed umbilicus. This figure gives but few specific 

 characters, and the description of the suture-line does not enable me to decide as 

 to its identity. As far as I can judge, the shell seems flatter, much more rapidly 

 increasing, and with a different suture-line. 



G. Stachii, Frech,^ and G. inexpectatus, Frech,^ are ovoid shells which have 

 broader backs and differ in the details of their suture-line, including a larger 

 central lobe. 



An American species, G. discoideus, Hall,^" comes very near to it in general 

 shape. It is, however, not ellipsoidal, and the suture-line is formed on a different 

 pattern ; the lateral lobes are closer, and not steeper ventrally than dorsally ; and 

 the lateral saddles are large, distant, and very deep. The shell appears flatter, 

 and to be usually more compressed in the centre. Its general facies is different, 

 especially in full-grown examples. 



1 1832, Muust., ' tJber Ciym. und Gon. Fichtelgeb.,' p. 18, pi. iv a, figs. 6 a— d. 



2 Ibid., p. 18, pi. iii A, figs. 7 a—c. 



■• 1851, Saudb., ' Verst. Eliein. Nassau,' p. 108, pi. v, fig. 4; and pi. x b, figs. 1—6, 8, 14, 15, 18, 

 19, 21, 23, 25. 



'' Ibid., p. 100, pis. X, X A, and x b. 

 5 1850, F. A. Ecim., 'Beitr.,' pt. 1, p. 27, pi. iv, fig. 15. 

 « 1860, ibid., pt. 4, p. 163, pi. xxv, figs. 17 a, h. 



^ 1837, Beyricli, ' Beitr. Verst. lihein. Ubergangsg.,' pt. 1, p. 42, pi. ii, fig. 7. 

 8 1887, T^rech, ' Zeitsch. deutsch. geol. GescU.,' p. 733, pi. xxviii, figs. 9, 11, 11 a. 

 •' Ibid., p. 733, pi. xxviii, figs. 10, a, h. 

 >o 1879, Hall, ' Pal. N. ¥.,' vol. v, ser. 2, p. 441, pi. Ixxi, figs. 1—13 ; and pi. ixxiv, figs. 4, 5. 



