POURTALESIA TANNERI. 



133 



The actinal side of P. Tanneri is also somewhat flatter than that of P. 

 hiijuncula. The smallest specimen of P. Tanneri collected measured 12 mm. 

 in length ; as in small specimens of P. laguncula, the test is more pointed 

 posteriorly and the proboscis less developed than in larger specimens. The 

 largest specimen collected measured 17 mm. in length, 9 ram. in greatest 

 height, and 8 mm. in greatest width, somewhat posteriorly of the actinal 

 groove. 



There was nothing either in the youngest or the largest specimen 

 examined, Figs. 189, 190, to show conclusively that the intercalated plates 

 in the interambulacral belt separating the trivium from the bivium belonged 



Fig. 189. Pourtalesia Tanneri. 



to the odd interambulacral area rather than to the posterior interambulacra. 

 These plates are marked by Loven as belonging to the odd interambulacrum 

 in Pourtalesia Jefreysi, Fig. 200. 



From the remarkable belt-like arrangement of the interambulacra which 

 is so well seen in an actinal view of Pourtalesia (PI. 56, fig. 1), Figs. 102, 

 191, 191 a , Loven 1 has suggested that the actinal groove of the Pourtalesia? 

 may have developed from the invagination of a similar belt at the anterior 

 area of the test. 2 It seems to me more natural to trace the course of this 



1 Loven, Pourtalesia, p. 30. 



2 The separation of the plates composing the sternum from the labium, so characteristic of the 

 Pourtalesia?, also occurs in some of the Clypeastroids, as in Encope, Clypeaster, Echinarachnius, and 

 Arachnoides. 



