19S 



THE VOYAGE OF H.M.S. CHALLENGER. 



primary spines, was an interesting discovery. The existence of a new species of Salenia 

 (Salenia hastigera) in the tropical Pacific has increased the number of living species of 

 that genus to four, and we now have a fair knowledge of a type which has played an 

 important part in the Echinoidal fauna of the Jurassic and Cretaceous periods. The 

 singular structure of the apical system of the genus, consisting of large plates soldered 

 together, and recalling the condition of the apical system in embryonic Echini, lias 

 led to important s}'stematic comparisons. 



Fig. 81. — Aerope rostrata, Wyv. Thorns. Seen in profile ; natural size. 



" A number of specimens of Ccelopleuriis maillardi were collected ; their examination 

 has thrown new light on the nature of the cap which tips the spines of the Arbaciadse. 

 In this genus it becomes developed to an extraordinary extent, four or five times the length 

 of the spine proper. The immense triangular and curved spines thus formed probably 

 served to raise the test as it were, on stilts, and enabled the sea-urchin to move with con- 



Fig. 82. — I'ourtalesia phiale, Wyv. Thorns. 



I actinal side (denuded) ; four times the natural size. 



siderable rapidity. We find in several of the species of Echinothuridse another form of 

 development of the tip of the primary spines. In Phormosoma hoplacantha, for instance, 

 the radioles of the actinal surface are tipped with broad conical shoes, which must give 

 to these soft-tested Echini a sufficient number of points of support to raise them 

 above the ground. This species is probably the largest sea-urchin known ; it must 

 have measured no less than 312 mm. in diameter when fully expanded. The 

 Echinothuridse, to which Phormosoma belongs, all have a more or less flexible test, made up 



