88 DEVONIAN FAUNA. 



Affinities. — Tr. reticulatum, Phillips, sp., differs in its flattened back and sides, 

 and in the closeness of its wave-like markings, which are twice as numerous. 

 Tr. pulcherrimum has distinct nodes upon the shoulder ; and in Tr. Vicarii the 

 back is more convex, and the ridges are fewer, and bear nodes which are elongated 

 longitudinally. 



Sandberger distinguishes, by a minute description of the external ornamenta- 

 tion, between three nearly alHed forms. It appears to me that the differences 

 mentioned in two of these, C. cancellatum, F. Romer,^ and G. quadrato-clathratum^ 

 Sandberger,^ are due only to the results of fossilization, and that it is also from 

 the same causes that they appear to differ from the present species. We find in 

 English examples of this group of shells considerable variation due to this cause. 

 The transverse striee are sometimes obliterated, sometimes they form regular 

 hollow tesserae, and sometimes the crossing of the markings " knot " the corners. 

 It appears hence that we have in our English form both the kinds of markings 

 described in the two German fragments ; and the form of the coil, the number and 

 prominence of the ridges, and the coarseness of the striation seem to be much the 

 same in all three. Possibly Sandberger's shell has rather fewer cross strise, as 

 they do not seem more numerous than the longitudinal series. It is to be noted 

 that the enlarged patterns of ornamentation given by Phillips give the impression of 

 closely arranged beads rather than of distant crossing lines, and hence it would be 

 little likely that the foreign authors would recognise the shell. The distinctions, 

 moreover, which F. Romer draws between his G. cancellatum and the present shell 

 seem due to the imperfections of Phillips's figure, in which the longitudinal lines 

 are represented as fewer and more rounded, and the rate of tapering greater, than 

 is really the case in his figured specimen. Sandberger's third species, G. tenuis- 

 quamatum^^ is distinguished by its much finer and more numerous striations, and 

 apparently by the almost entire absence of swellings upon the shoulders. 



The fragments which Portlock* identifies with this species are clearly shown 

 by his figure of the enlarged sculpture not to belong to it. 



The Bohemian species which is most approximate is Tr. trochoides,^ Barrande, 

 which has a circular section and apparently coarser ornamentation, and has 

 stronger and straighter transverse bulges, which slope obliquely backwards across 

 the sides and back, without vanishing in either direction. 



1 1844, F. Eomer, ' Ehein. tjbergangsg.,' p. 81, pi. vi, figs. 4 a — c. 



2 1852, Sandberger, ' Verst. Ehein. Nassau,' p. 138, pi. xv, fig. 6. 



3 Ibid., p. 138, pi. XV, fig. 7. 



* 1843, Portlock, ' G-eol. Eep. Londonderry,' p. 384, pi. xxviiis, figs. 5 a, i. 



5 1865, Barrande, ' Syst. Sil. Boheme,' vol. ii, p. 116, pi. xxix, figs. 16—26, Et. E. 



