196 



SILURIAN TRILOBITES. 



pared with //. Bowmanni and its allies ; but this is a true lllanus, and does not need 

 comparison with that species. I give the description, revised, from ' Decade 2/ 



General form oval ; length to width, as 17 : 10 ; the semioval head and semicircular 

 tail nearly equal in length ; the thorax about two thirds the length of either, and strongly, 

 but not deeply, trilobate. The axal furrows are carried less than half way up the 

 head, and but a little way down the tail (except in young specimens, fig. 16). 



Head more depressed than a quarter of a sphere, and regularly convex, not gibbous 

 behind (our fig. 13 is too much depressed, being crushed a little). It is divided into 

 three nearly equal parts by the short and slightly converging axal furrows, which turn 

 out again, and then cease at about the level of the top of the eye. The latter is of 

 moderate size, gently lunate and narrow, bounded beneath by a slight furrow, and placed 

 fully its own length from the posterior margin, which shows no trace of a neck-furrow, 

 within or without the crust. The facial suture is divergent above the eye, and slightly so 

 below it, so as to cut the margin beneath the most prominent curve of that organ. The 

 rostral shield (fig. 14) is shuttle-shaped, more than twice as wide as long, and produced 

 into an angle below, where the labrum, which we do not yet know, would fit to it. 



Thorax of ten narrow segments, the axis well marked, gently convex, and subfusi- 

 form; wider than the pleurae in the forward segments, in the last only equal to them. 

 The fulcrum is very near the axis in the first segment, and in the last placed scarcely 

 more than a third along the pleurae, which have a distinct facet and oblique ends. The 

 front pleurae bend down and a little back ; the hinder ones are straight, only bent downward. 



Tail semicircular and moderately convex, chiefly so toward the margin, which descends 

 abruptly, but rather flattened along the anterior two thirds. The axis is 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, but are not truncated as in many species ; so 

 that the outhne of the tail is tolerably semicircular, and straighter in front than in 

 many forms of the genus. The fascia is of even width all round, and not very broad ; 

 it is rather finely striate. The tail, too, has oblique ornamental lines round the margin. 



In the tail of young specimens the axis is marked out nearly all round (fig. 16), and 

 extends three-fifths down the tail, which is also flatter. 



Far. j3, Involutus. — ' Decade 2, Geol. Survey,' pi. ii, fig. 8. 



The axis in some specimens is so much narrower, and the tail-margin so much more 

 incurved, that the specimens possessing these characters might well pass for examples 

 of a new species. The axis is truly very narrow, and the fulcrum more remote, as usual 

 in all such cases. I find the same form in Dr. Wyville Thomson's Cabinet, from the 

 Ayrshire district, and note it under a varietal name, not much doubting that, when we 

 know the perfect form, we shall find this variety a true species, or, at least, one of those 

 constant forms which botanists call sub-species, — a very useful term. 



