ILLJENUS. 



193 



eye, the convexity of the head between the eyes, and the proportionally longer tail-piece, 

 our fossil differs from the ordinary //, crassicauda. A nearer approach is made in these 

 respects to Dr. Volborth's new variety or species //. {crass.) Dalmani, Volb. ; but I will not 

 unite it with that form, which has larger eyes, nearer the glabella — a longer head, and 

 squarer head-angles. The species is so near to these two Russian forms, that I give a 

 woodcut of both, more especially for the purpose of showing the structural points 

 gained for us by the acumen of Dr. Volborth, in the work cited in the foot-note. 



We have but one specimen, rolled up, of this fine species, for the use of which I am in- 

 debted to the Irish Geol. Survey. At first sight it was naturally taken for //. Boiomanni, the 

 common Lower Silurian form ; and it occurs in beds of the same age. To that species, too, it 

 bears much resemblance in the shape of the head, position of the eyes, and the general aspect. 



But it is a true Illcenus, with 10 rings — not a Bysplanus ; and its alliance is with the 

 Swedish forms of //. crassicauda, Wahl., and //. Balmani of Volborth. crassicauda 

 has been often quoted from Britain, but always erroneously ; whether this may be an 

 extreme variety of that northern species, uniting the above-mentioned forms, I am not 

 quite sure ; but I shall try to point out its proportional differences, which are nearly 

 all the characters we have to rely on ; at least, in tlie great majority of the species of this 

 very uniform genus. 



Our coiled-up specimen, with the crust preserved, measures rather more than three 

 inches in breadth, and when uncoiled must have been four inches long. Of this length 

 the large semicircular head, more than a quarter of a sphere in convexity,i is two and 

 three quarter inches long, obtuse in form, but regularly convex, and three inches broad. 

 The glabella occupies less than a third of this breadth, and is defined only by short axal 

 furrows, strongest behind, which do not reach one third up the head. The prominent 

 eye is placed as far from the glabella as the width of the latter, and very close to the 

 hinder margin — not its own length from it. The facial suture beneath it turns sharply 

 outward. In front of the eye its course is not known. The cheeks slope gradually 

 down, not abruptly, and are greatly rounded off at the angles — more so than in kindred 

 species. The chief convexity of the head is more than midway up — not near the vertex 

 as in //. crassicauda, which, seen from above, has a shorter head and less rounded angles 

 (Volborth, 1. c, pi. ii). 



The thorax is much shorter than the head, and rather shorter than the tail, the pro- 

 portions of the three being nearly as 4 : 2^ : 3. This may probably vary in some indi- 

 viduals, and is sure to be different in younger specimens, which would have the tail shorter. 



dorsal vessel (see woodcut 49) must be the cast of an annulated or saccate intestine. It seems to me all 

 but impossible that the contents of the heart should be preserved in a solid state ; but the ingesta may easily 

 be RO, if I am right in supposing the food to be silty matter. 



1 A precise English term conveying this idea— parallel to the terms spherical, hemispherical — 

 is wanted. Will any Greek scholar supply it? Tetartospherical is a long word to use, and I hate verba 

 sesqui pedalia ; still, we require a term for a shape very common iu the carapaces of Crustacea. 



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