254 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



plate as viewed from the inside of the pinnular forms approximately a quarter 

 of a circle, the outer edge as viewed ventrally is almost a right angle, the apex 

 of this angle coinciding with the apex of the notch between the proximal extension, 

 which, viewed laterally, appears to be very narrow, but viewed ventrally is seen 

 to be quite deep, and the median angle. 



The covering plates, which are circular with the outer half of their circum- 

 ference bordered with long spines, are attached to the outer side of the proximal 

 angle. 



Toward the base of the pinnules the side plates degenerate, the distal portion 

 gradually separating off and appearing as a straight rod directed outward and 

 slightly distally, and the remaining portion gradually becoming an irregular 

 calcareous film which grows smaller and smaller until it disappears. 



Relatively large covering plates with unfinished edges are found in the lappets 

 aft«r the side plates have become very minute. 



Stenometra quinquecostata. — In their simplest form the side plates are squarish 

 in lateral view; the proximal edge is usually straight, sometimes with the median 

 third slightly concave; the lateral edges are slightly incurved, the distal overlap- 

 ping the proximal of the succeeding plate; the outer proximal corner is broadly 

 rounded; the outer distal corner is rather sharply rounded; in the middle of the 

 distal edge is a broadly rounded angle the distal point of which is on the same 

 level as the outer points of the rounded proximal and distal angles from which 

 it is separated by shallow rounded incisions, that on the distal side being deeper 

 than that on the proximal. Toward the distal end of the pinnule this form becomes 

 slightly modified by a deepening of the notch between the median angle on the 

 outer border and the distal process, which becomes longer and narrower, and 

 a deepening of the concavitj^ of the distal lateral border. 



The same modification takes place toward the base of the pinnule, where it is 

 carried farther, so that both the proximal and the distal sides become strongly 

 concave and the distal process becomes very long and thin. At the base of the 

 pinnules the side plates become lower and lower and finally disappear ; the cover- 

 ing plates, though the}'^ become smaller, persist to the junction with the arm. In 

 direct ventral view the outer edges of the side plates are seen to be curved in such 

 a way that they form about a quarter of a circle; in dorsal view the lower edges 

 are parenthesis shaped with the convexity outward. The distal edge of the side 

 plates is thickened beneath the insertion of the covering plates, and in the distal 

 process. 



The almost circular covering plates are attached to the outer border of the 

 side plates just within the proximal outer angle. 



The perisome between the side plates and the pinnulars is completely incased 

 by large and filmy plates. 



There are no deposits in the tentacles. 



Stenometra etistata (figs. 820-822, p. 392). — The side and covering plates in 

 this species resemble those of &. quinquecostata. 



Stenometra diadema. — The side and covering plates are essentially like those 

 of S. quinquecostata. 



