464 PYGOCEPHALUS COOPERI 



of the segmented part of the body, the four pairs nearer the quadrate 

 disk being directed towards that end of the body, while the others 

 pass more directly outwards. 



The semicircular disk (c) is traversed by a strong transverse de- 

 pression about the middle of its length, which divides it into a wide 

 proximal, and a narrower distal portion. The latter is convex in the 

 direction of the long axis of the body, and presents a little tubercle 

 on each side of the median line close to the transverse depression. 

 The periphery of the distal portion has the form of a curved ridge, 

 separated by a corresponding groove from the rest of the upper sur- 

 face, and by another groove from the line of junction of the upper 

 and lower surfaces (e). On clearing away the matrix as far as \\'as 

 practicable, I found the under surface of this part to be far more 

 convex than the upper, and to present a transverse line apparently 

 indicating the boundary of a segment. In consequence of its con- 

 vexity inferiorly, this portion of the body has a thickness of as 

 much as |ths of an inch. 



The pro.ximal portion of the semicircular disk appears to be some- 

 what crushed ; it is divided by two well-marked longitudinal depres- 

 sions, which converge from the ends of the transverse depression into 

 a central lobe, narrower proximally than distally, and two lateral ones, 

 whose wider extremities are turned in the opposite direction. 



The proximal half of the quadrate disk (a) presents two convex 

 lateral eminences, separated by a slight depression, which, like the 

 distal half, is obscured by portions of the matrix. The larger 

 lateral appendage (2', 2") attached to the distal half, is, on the right 

 side, composed of a short, wide, basal articulation, with which a 

 quadrate joint, produced into a spine at its outer and distal angle, and 

 presenting a convex outer curve, is articulated. Apparently con- 

 tinuous with the under edge of this joint, is a flat, broad plate {2"), 

 with an oval, distal contour, and presenting what appear to be 

 traces of a fringe of setse on one side. Regarding the quadrate 

 joint and this plate as one part, they form a scoop-like scale, \ inch 

 long by \ inch wide. Lying in the hollow of the scoop, and articu- 

 lated either with its base or with the basal joint itself, is a fusiform 

 mass {2') divided by two constrictions into three joints, the middle one 

 being shortest and subquadrate, while the inner and outer are conical. 

 The outermost passes into a cylindrical multiarticulate filament, whose 

 extremity is buried in the matrix. A longitudinal groove furrows 

 the outermost articulation and is continued on to the filament, giving 

 rise to an appearance of division — but I believe this to be acci- 

 dental. 



