2 



BRITISH FOSSILS. 



this point. The anterior segments are bent down slightly, and also a little 

 backward at the fulcrum, while the posterior ones are only bent down- 

 wards. Tail semicircular and moderately convex ; the axis indistinctly 

 marked out by two deep impressions, which sometimes form short rapidly- 

 converging furrows ; the upper corners are bent sharply down beyond the 

 fulcrum, in order to pass freely under the thorax-rings in rolling ; the 

 incurved under portion is concentrically striate as usual, not broad nor 

 indented by the termination of the abdominal axis.* General surface 

 of the head not well known ; a few wandering lines appear on one spe- 

 cimen near the margins of the cheeks ; the thorax rings are unorna- 

 mented : the general aspect is smooth. 



Variations. — The axis in some specimens is narrower in proportion to 

 the pleurae (fig. 7). 



Affinities. — This species requires close examination to distinguish it 

 from several others. /. Bowmanni has only nine rings to the very short 

 thorax. The ///. crassicauda has a head very gibbous behind, and 

 longer than the tail ; the eyes placed at less than their own length from 

 the neck margin, and remotely from the axal furrows ; the tail, which 

 has the axis distinctly marked all round, is scarcely longer than the 

 thorax, and the latter has its axis broader and flatter than in our spe- 

 cies ; the fulcrum of the pleurae not close in front, and at a full half 

 behind ; the incurved under portion of the tail is far broader and less 

 concave, and its lines of growth meet at an angle in the middle ; this 

 last portion is, however, not always visible for the purpose of com- 

 parison. The whole surface, too, is covered with strong, sharp lines, 

 which are certainly not conspicuous upon III. Davisii, though the latter 

 is not quite smooth. ///. perovalis (Murch ) is a more elongate oblong 

 species ; the axis of the tail is very narrow, more so than in our nar- 

 rowest variety, extends one-third down its length, and is circumscribed ; 

 the incurved portion of the tail is broad. It is not at present justifiable 

 to connect even this latter with crassicauda, although it agrees more 

 nearly with it than with any other species. 



History. — Apparently it was not known till Professor Sedgwick and 

 myself collected it at Bala, in what was then considered a peculiar band 

 of limestone, and it was too hastily identified with the 111. Bowmanni 

 (Mem. Geol. Surv., vol. ii., part 1, pi. 8, figs. 1, 2, 3), and as such pub- 

 lished in the list of Welsh fossils given in the 1st volume of the Geol. 

 Journ., p. 20 : the Rhiwlas locality belongs to the present species. Mr. 

 D. Sharpe, in noticing the probable occurrence of III. crassicauda in 

 America, states that this fossil is found at Rhiwlas ; the present species 

 only is intended, and Mr. Sharpe differs with me as to its specific value. 



* This indentation is frequently to be seen in Asaphus ; it is strong in Illcenus 

 crassicauda, Dalm. 



