MONOGRAPH OF THE EXISTING CRINOIDS. 263 



near the proximal border; this is perfectly straight and perpendicular to the side 

 of the pinnular, running to the attachment of the covering plate; the other is 

 midway between this and the distal border; it runs parallel to the first as far as 

 the hole allowing for the protrusion of the sacculus, about the outer edge of which 

 it turns, running to the distal side of the plate ; in a direct ventral view of the plate 

 these two ridges appear as two thin processes of considerable height. 



The sides of the covering plates, which are semicircular in their outer half, 

 gradually converge in almost straight lines to the relatively narrow base. 



Pcecilometra scalaris (figs. 849-855, p. 412; and fig. 1169, pi. 27). The side 

 plates are broader than high ; the outer half of the proximal border is produced in 

 the form of a proximal process with parallel sides, which is directed slightly out- 

 ward ; the outer edge of this process is parallel with the edge of the inner half of the 

 proximal side ; the edge connecting the two in lateral view is at an angle of about 

 135 with each ; on the outer edge directly over the middle of the base is a prominent 

 angle, usually of about 90 ; the outer distal corner of the plate is produced in the 

 form of an acute angle of about 45, which lies at the same height as the outer 

 proximal angle. Between the proximal outer angle and the median angle the outer 

 border forms a reentrant obtuse angle, the apex of which, just proximal to which 

 the covering plate is attached, is from one-fourth to one-third of the distance from 

 the median angle to the proximal outer corner. The outer edge between the median 

 angle and the distal angular process is excavated in the form of a semicircle. In 

 this excavation is a plate which is unusually well developed, shaped like a thick 

 crescent, with the concave side directed toward the base of the side plate, so that a 

 large oval or circular hole is left between it and the border of the side plate. Be- 

 tween the deepest portion of the excavation between the median angle and the 

 distal outer corner and the middle of the distal border of the side plate, and equi- 

 distant from both, is a rather small circular opening, through which the outer 

 portion of a sacculus protrudes. The distal border of the side plates is very 

 slightly and evenly convex ; in direct ventral view the edge of the proximal third 

 of the plates is straight and parallel to the base ; the remaining portion is excavated 

 in the form of a semicircle; the side plates are slightly convex outwardly in the 

 horizontal as well as in the transverse plane. As a result of this the opening for the 

 sacculus is directed diagonally outward and somewhat distally; the inner edge 

 of the side plates, adjoining the pinnulars, is straight and bears directed inwardly 

 two rather long processes, one just within the distal end, the other midway between 

 this and the proximal end, which are the two high internal ridges seen in end view ; 

 on the inner side of the side plates are two relatively high ridges; one of these, 

 arising near the proximal end of the base, runs directly upward perpendicular to 

 the latter to the attachment of the covering plate ; the other, halfway between this 

 and the distal border, runs perpendicular to the base as far as the hole for the 

 passage of the sacculus, about the distal side of which it turns, and runs outward 

 to the distal border of the plate. The sacculi occupy the entire region between the 

 ridges on adjacent plates, lying over the sutures between them. 



The covering plates are usually slightly longer than broad, broadly oval, 

 broadest at about the middle, with the angle between the flattened base and the 



142140 21 Bull. 82 19 



