ISOTELUS. 163 



heard of, for supposing the great /. megistos of American authors to be so large as the 

 cast sold for lecture-purposes would indicate. Twenty-one inches is not too long for the 

 largest Paradoxides, but is far too much for any species of Asaphus. 



" General shape oval-oblong, with the sides rather straight ; the head and tail nearly 

 equal, and both subtriangular ; the head pointed, the tail more obtuse at the tip ; the 

 surface is convex, a line taken from the snout to the apex of the tail being a regularly 

 convex one, uninterrupted by neck-furrow, depression or convexity of the smooth and 

 even body-rings, or by furrows on the axis of the tail. The axal furrows are very obscure 

 in the head ; they are neatly marked, but shallow, along the body, and only very faint 

 along the tail; all the surface is smooth, and the sides are strongly deflected, but not steep. 

 The head has the shape of a broad and pointed Gothic arch, the breadth at base being 

 to the length as three to two ; the margin is very narrow and flat, not at all recurved ; 

 the facial suture, forming a broad ogive arch in front, runs for some distance close within 

 and parallel to the front margin ; and beneath the eyes, — which are large, placed near the 

 glabella, and rather behind the middle of the head — the suture curves gently out and 

 cuts the hinder margin midway ; the head-angles are blunt-pointed, not rounded. On 

 the under side of the cheek, near the angle, is a convex space containing an oval 

 depression, which receives the apices of the front pleurae in rolling up (fig. 6 ; see also 

 fig. 5 for the cast of this depression on the matrix). The labrum (fig. 7) has a narrow 

 base, then a strong constriction, and thence the sides are parallel ; its apex is deeply 

 furcate, the parallel forks occupying nearly half the entire length of the organ. Body- 

 rings smooth, rounded at the apices, deflexed at the fulcrum, which is placed rather 

 beyond one third, and with a broad strong groove. Tail subtrigonal, with straight sides 

 and rounded blunt tip ; the faint axis rapidly tapering, broad-conical, and reaching three 

 quarters the length ; sides quite smooth. In young specimens (says Professor Hall) the 

 tail is more pointed, and exhibits eight faint articulations ; in older specimens these 

 increase in number, but the crust presents many traces of them when viewed from within, 

 and they are often distinct (Hall, Pal. New York, loc. cit., p. 231). Dr. Burmeister also 

 calls attention to this character, which is, indeed, no more, as we shaU see by and bye, 

 than occurs in most of the smooth- shelled genera, the lobes being only obscured, not 

 destroyed, by the even contour of the crust. Illcenus shows this well, not only on the 

 axis, but even in the inflated glabella. I have not seen internal lobes to the glabella in 

 this species, but Dr. Burmeister figures them as strongly expressed." 



Varieties. — If I am right in connecting the two forms above given — viz., A. platy- 

 cephalus, Stokes, and /. gigas, there are two very distinct varieties of this species — the 

 one, A. platycephalus, with " broad form and with small eyes ; the other, of elongate 

 form, and with variably large eyes, to which nearly all the above synonyms belong." I 

 have quoted the best figures of these from the 'Geology of Canada,' (1863), by Sir W. 

 Logan and E. Billings. But I have no difficulty in referring these differences to sex, 

 the common form, I. gigas, being the c? • 



