4 



BRITISH FOSSILS. 



nearly to the tip. A rather narrow ring or rim surrounds the apex 

 and sides, terminating abruptly near the base on each side in what 

 may be called an auricle, followed by a deep notch c, above which 

 the ascending processes a, a, take their origin. The apex of the 

 hypostome is truncate, the corners angular or even mucronate. A 

 distinct sulcus separates the border all round, and within this there 

 is a short oblique furrow on each side. Its whole surface is closely 

 scabrous (fig. 15*) ; the convex portion has besides scattered larger 

 granules. The organ is hollow when viewed from the inner and 

 under side, and the structure there observable is such as has been 

 often described.* These are two ascending processes, a, a, rising 

 from the ends of the basal or front margin, and directed obliquely 

 backwards ; and on the sides, h, 6, the inflated broadly triangular 

 portions characteristic of the genus. These triangular curved plates 

 give the appearance of thickness on viewing the organ from the 

 side (fig. 12), but the general surface on the inner side is concave, 

 answering to the great convexity of the outer side. Thorax much 

 longer than the head, but narrower, and for most part of it parallel 

 sided, of 11 gently convex rings which are very minutely scabrous ; 

 the axis is narrower than the glabella, of liearly equal width 

 all the way down, but scarcely so wide as the pleurae. These are 

 linear and directed straight outwards for two thirds their length, 

 then curved a little backwards and tapering to a sharp point. The 

 fulcrum, placed at about one third, is of singular structure, — a 

 small semi-oval piece (fig. 8, a) is attached to the posterior edge of 

 each pleura, and against this piece abuts a similar tubercle (b), placed 

 on the front edge of each, and the two pieces, forming together a 

 narrow oval tubercle, are insulated by a deep sulcus from the body 

 of the plem-a, which is also constricted and furrowed across at this 

 point, so as to have the outer and pointed portion (c) quite distinctly 

 separated from the small inner one. The latter {d) is very strongly 

 divided into two tumid lobes by a short oblique sulcus, and just 

 beyond the constriction the outer portion rises into a stout boss, 

 (fig. 9, e) giving the tri-tuberculate form characteristic of the genus. 



* M. de Barrande, Neues Jahrbuch (1847), 389, has given a full description of the 

 hypostome of Cheirurus. He describes the ascending processes a, a, (Fliigel), as bent 

 upwards at right angles to the surface of the organ, and uniting with the upper crust along 

 the line of the dorsal or axal furrow, with a broad base of attachment, reaching from the 

 upper to the middle glabella furrow. In Phacops it has nearly the same position. He 

 also describes a second organ, of the same size and shape, but less convex in all its parts, 

 lying immediately behind the hypostome, between it and the upper crust of the head. 

 This organ he calls epistoma ; and he has seen it both in Cheirurus insignis, and a species 

 of Phacops. It has never yet occurred to our observation. 



