MONOGRAPH OF THE EXISTING CMNOIDS. 



251 



the ambulacral groove. The outer part of the side plates is strongly incurved 

 toward the median line of the pinnule; the small plate between the distal and 

 proximal processes lies in a semicircular area bounded proximally by a blunt angle 

 on the outer side of the broad proximal process. 



Pectinometra Havopurpurea. — The perisomic plates resemble those of P. car- 

 duum. 



Family THALASSOMETRID^. 



Ptilcyinetra macronema (figs. 857, 858, p. 412). — In lateral view the outline of 

 the side plates is almost square, the distal and proximal edges, which are parallel, 

 leaning slightly distally; about a third of the distal edge is cut away in a semi- 

 circular excision which is just above the middle, so that the outer distal corner, 

 which is slightly produced and overlaps the outer proximal angle of the succeed- 

 ing plate, is narrower and more pointed than the broadly rounded lower distal 

 corner; the corresponding notch on the proximal border of the plate is much less 

 deep. The sacculi are relatively small, and sometimes absent, in which case the 

 distal and proximal borders of the plates are straight, and the former overlap 

 slightly the proximal borders of the succeeding plates. 



From this simple type of side plate there is derived a more specialized type 

 which in all essentials resembles the side plates of the Calometridse. On the distal 

 half of the outer border a curved incision makes its appearance, so that the outer 

 distal portion of the plate becomes restricted to a narrow process directed outward 

 at a small angle to the pinnulars, and slightly curved outward. The proximal 

 half of this outer edge, proximal to the curved incision, is straight and supports 

 the covering plate. 



On the inner side the surface of the distal process is thickened into a promi- 

 nent keel. Tangent to the incision on the proximal border a strong rib runs across 

 the whole side of the plate perpendicular to the base, which is produced down- 

 ward in the form of a long, straight spine lying within the groove of the pinnular. 



There is no small plate or spicule in the excision on the outer edge of the 

 side plates such as is found in the Calometridse. 



The structure of the plates is much coarser than in the Calometridse. 



The covering plates, which are attached to the outer end of the long straight 

 rib on the inner side of the side plates, are relatively slightly smaller than those of 

 the Calometridse, and almost circular in outline, though frequently slightly elon- 

 gated in the direction of the chief axis. 



Ptilometra 7Jiulleri — The side and covering plates in this species do not differ 

 essentially from those in Pt. macronema. 



Asterometra anthus. — In direct lateral view the side plates are seen to have the 

 outer and inner sides, which are parallel, slightly longer than the proximal and 

 distal sides, which are also parallel; the distal edge of each overlaps slightly the 

 proximal edge of the succeeding; the outer border is very slightly depressed in the 

 middle ; from the proximal inner angle, which may be slightly produced proximally, 

 a broad process, in width equal to about one-third the length of the inner border, 

 extends inward and slightly distally over the inner surface of the pinnular for a 



