BRITISH FOSSILS. 



s 



shows any convexity ; it is narrow, not occupying above one sixth 

 the width of the thorax, and consequently is much narrower than 

 the glabella ; its rings show the usual division into two parts, an 

 external arch and an articulating front portion. The pleurae are 

 truly flat, and only marked with a very faint diagonal furrow, but 

 at their extreme end they are a little bent down and strongly 

 indented (fig. 5). The fulcrum (fig. 5, a) occurs immediately before 

 the tip. Tail rounded, truncate, less than a semicircle, concave, 

 except the axis ; the latter is moderately convex, narrow, and 

 tapering to a point which reaches the margin ; it is annulated by 

 seven or eight faint rings, which are indented in the middle. The 

 sides of the tail have seven or eight farrows, nearly reaching the 

 margin ; the upper one is straight or nearly so, the ends of the rest 

 are strongly curved backwards. The very narrow margin of the 

 tail is bent down vertically, so as to be invisible in a direct view ; 

 a small portion only of it is seen at 6 in fig. 1*, where the tip of the 

 tail is decidedly recurved. 



Variations — In many specimens the pendant ears are not so long 

 as in our figured example, and consequently the posterior angles are 

 more obtuse. This is particularly the case with those from the 

 mining district of Shelve and Middleton, in Shropshire ; these spe- 

 cimens have also smaller head spines, and the ears are much 

 smaller, and are truncated so as hardly to reach back beyond the 

 first or second thorax ring. This variation may be designated by 

 the name corndensis, and if at all common (we have only seen it in 

 specimens from one locality at present) may probably be charac- 

 teristic of the female. We have figured a young specimen of it at 

 fig. 2, and the head, magnified, fig. 6. The fringe in this specimen is 

 scarcely concave, and the collocation of the pores into rows very 

 indistinct toward the sides. 



Affinities. — It is sufiiciently distinct as not to be easily con- 

 founded with the common species, T. concentricus, Eaton, (known 

 better in England as T. Caractaci, Murch.) The great size and pen- 

 dant form of the large head- wings easily distinguish it from that 

 species, and also from the T. fimhriatus, Murch. The concave cha- 

 racter of the fringe distinguishes it from T. radiatus, Murch., which 

 too has a square form of head, fr'om the enlargement of the upper 

 corners of the fringe, and divergent not parallel spines. Its nearest 

 ally, to which, indeed, several authors have referred it, is T. granu- 

 latus of Wahlenberg and Dalman. Good means of comparison, 

 however, are now given us by the accurate figures of Loven, who of 

 course has access to the very specimens described by the Swedish 



