ASAPHTJS (ISOTELUS) INSTABILIS. 17 



to eyes, in front of which they are obsolete. Eyes situated close to glabella at 

 about two-fifths of its length, and therefore nearer posterior than anterior margin 

 of head-shield. Eye-lobes of moderate size, rounded, prominent, upturned. Facial 

 sutures unite in regular curve in front of glabella, curve at first strongly outwards 

 and then inwards to eyes, behind which they bend again sharply outwards to cut 

 posterior margin at angle of about 35° — 45° at less than half the distance between 

 glabella and genal angle. Free cheeks triangular, convex, with slightly flattened 

 lateral border and genal angle furnished with a short backwardly directed spine. 



Thorax with cylindrical axis as wide as the pleura?. Pleura? consist of inner 

 horizontal straight portion and outer extra-fulcral portion bent downward and 

 slightly backward. Fulcrum situated nearly halfway out at junction of inner 

 and outer portions. Pleural groove strong, deep, wide, extending only about two 

 thirds the length of pleura. Extremity of outer portion of pleura flattened, 

 rounded, or subtruncate, with " Pander's organ " near front margin represented by 

 small oval tubercle. 



Pygidium broadly parabolic (or semicircular in immature individuals), feebly 

 convex ; front margin slightly arched forward ; lateral angles obliquely truncated 

 by inclined articulating facet. Axis about one third the width of the pygidium at 

 front end, and extending fully three fourths its length ; weakly convex, tapering 

 rather rapidly, especially at first, to a blunt slightly raised apex. Faint traces of 

 several ampliations on axis (visible in casts). Lateral lobes slightly bent down, 

 with faint traces of several pleura? (visible in casts). Fulcrum situated at about 

 one third of the distance from axis to lateral margin, with strong oblique groove 

 behind it. Fascia about one fifth the length of pygidium, concave, with the stria- 

 meeting behind axis at obtuse angle and extending slightly over to convex surface of 

 pygidium. 



Remarks. — In every collection which I have examined there are head-shields, 

 pygidia, and portions of thoraces (with occasional hypostomes) which are referable 

 to some species of Asaph us allied to A. gigas, De Kay, A. scutalis, Salter, and 

 A. rectifrons, Portlock, and yet showing points of difference which forbid our 

 placing them in any one of these species. Nicholson and Etheridge were doubtful 

 if they could be referred to A. gigas (M., fasc. ii, p. 153). The head-shields 

 plainly indicate the sub-genus Isotelus by the absence of neck furrow, occipital 

 furrow, and neck-ring. The free cheeks are also typically those of Isotelus. 

 Several small specimens — probably immature — from Dow Hill in Mrs. Gray's 

 collection consist of head-shields with several thoracic rings attached, or of 

 pygidia with several segments of a similar thorax, so that we are able to recon- 

 struct the complete individual. 



All these specimens appear to have been previously attributed to A. gigas ; but 

 the head-shields differ from A. gigas by being broader and more transverse, by the 

 more backward position of the eyes, by the posterior branch of the facial suture 



