1912] The Ottawa Naturalist. 81 



bulaereals become marginals. The adambulacreal plates are 

 rather small, oval, placed with the long axis diagonal to the 

 axis of the arm. On the most perfect arm there are 16 of these 

 plates on each side of the groove, not counting the proximal 

 and distal plates. 



The ambulacreal ossicles are small, arranged alternately. 

 On this specimen they are mostly displaced. The plates on 

 the disk between the rays are few and small. The marginals 

 are small, rounded, and do not appear to bear spines, but this 

 appearance may be due to poor preservation. 



Locality. This species is described from a single specimen 

 collected by the writer in the Madison Limestone at Spring 

 Canon in the Ruby Mountains, near Alder, Montana. The type 

 is in the Carnegie Museum, Pittsburgh, Penna. 



Explanation of Plate. 



1. Upper left-hand figure. Palceastert wilsoni Raymond. 

 All that is preserved of the specimen. The brachiopods are 

 Rhynchotrema inaquivalve . One-half larger than natural size. 



2. Upper right-hand figure An enlargement of the ray 

 which extends downward to Fig. 1 , to show the character of the 

 large plates along the top of the arm. X 3. 



3. Central figure. The most perfect arm, viewed from the 

 side. Notice the two rows of large marginals, the overlapping 

 triangular plates above them near the middle of the arm, and the 

 flatter and more nearly square plates to the right, nearer the 

 disk. X 3. 



4. Lower figure. An enlargement of part of the specimen, 

 to show the ambulacral plates in the ray pointing upward, the 

 madreporite, and the small, irregular plates along the top of 

 the arm. The arrow points to the two ambulacrals which are 

 pierced by vertical pores. X 3. 



All the photographs were made at the Geological Survey 

 of Canada, and are published by permission of the Director. 



