206 



SILURIAN TRILOBITES. 



above the large eyes, which, in Miss Jukes' specimen, are 2\ inches apart, measuring to their 

 outer edges. The furrows terminate above the eye (but only on the inner surface of the crust) 

 in oval depressed spaces, the nature of which is not yet known, but which probably represent a 

 pair of the curious glands (?) described under the genus P^aco/;-? (see under F. caudatus, p. 52). 

 Beneath the eye, a strong rounded fold or long cushion {puhillus) of the crust supports that 

 organ (the fold is not seen in young specimens), and this cushion is subtended by a deep 

 furrow, that encircles the whole of the lentiferous surface of the eye. The latter is much 

 curved, and quite smooth, and has a thick cornea. The encircUng furrow ends in front in a 

 broad deep depression (PI. XXVII, fig. 16) which indents the head deeply just at the origin 

 of the facial suture above the eye ; the suture from thence curves boldly S-fashion out to 

 the margin, which it cuts on a line level with the outer edge of the eye. Beneath the eye, 

 the suture again turns sharply outwards, cutting the posterior margin in the same vertical 

 line. The eye-lobe {palpehra, Dalman) is very convex and inclines downwards. The inferior 

 fold or eyelid, if so it may be called, is deeply punctate in our specimen, but without any 

 of the wavy lines which cover other parts of the head. (Decade 2, Geol. Survey.) The 

 cheek, outside the eye, is tumid, and the angle quite blunt. The neck-furrow is always 

 obscure in the species of this genus. The whole head is covered with the peculiar 

 squamous lineation characteristic of the family Asaphida ; its peculiarities are more 

 particularly described below. 



The epistome beneath the head is broad, and very strongly striated transversely by 

 sharp squamous striae. (See Decade 2, Geol. Survey, pi. iv, fig. 7.) We have omitted 

 to figure it again in our plate. The rostral shield is very distinct, and often found separate 

 in this and other species of III anus ; it is a transverse piece, long-oval in shape in our fossil 

 (many of the species have it rhomboidal) and with pointed ends ; the rostral suture is very 

 distinct, and the sutures that separate it from the side-pieces equally so. The labrum 

 should be looked for. It is strange it should not be yet known in collections. 



Thorax of 10 segments, arched forward, especially the front ones, and having the 

 three lobes just indicated by a very sligjit furrow where the fulcrum is placed, the sub- 

 fusiform axis occupying more than two thirds the entire width of the thorax. The fulcrum 

 is, of course, far outwards ; and is formed by a short forward bend of each pleura, which 

 then continues in the general direction, is sharpened anteriorly for rolling, and curves 

 forward at its blunt end ; the foremost pleurae are rapidly shortened, the whole of the 

 lateral portions of the great head projecting beyond them (fig. 2). 



Tail, in the young state, transverse broad-oval ; and in the adult not much more 

 than a semicircle. The upper corners are truncated, the facet strong. There are no 

 indications at all of the axal lobe, and the whole tail is regularly and gently convex 

 from side to side, and from front to back ; and is not nearly so hemispherical as in the 

 next species. The incurved portion (caudal fascia) is broad and not so convex as in 

 the next species, //. insi(/nis. It is, however, remote from the inner surface of the crust, 

 leaving a hollow channel, which is conspicuous enough in broken specimens. 



