174 HAWAIIAN AND OTHER PACIFIC ECHINI. 



Ar^osoma. 



Mortensen, 1903. " Ingolf " Ech., pt. 1, p. 53. 

 Type-species, Calreria fenestrate/, Wyville Thomson, 1872. Proc. Roy. Soc. London, 



XX, p. 494. 

 (Including Calveria and Hapalosoma of Mortensen.) 



Actinally this genus is not essentially different from Asthenosoma, but 

 seen from above the difference is quite marked. In Araeosoma none of 

 the abactinal primary spines are encased in loose skin-sheaths. There 

 are at least 25-30, and sometimes several hundred, primary tubercles 

 which are much more conspicuous than the rest and their areolae are cor- 

 respondingly large. The coronal plates are also much higher than in 

 Asthenosoma, but the texture of the test varies much in the different species. 

 This is the largest genus of the family, but although the species show 

 sonTe tendency to an arrangement in three or four groups, we have failed 

 to find any satisfactory characters by which such groups may be constantly 

 distinguished. We can hardly believe that the texture of the test, the 

 relative width of ambulacra and interambulacra, or the relative number 

 of ambulacral and interambulacral plates, are any better generic characters, 

 taken by themselves, than the color or the pedicellarioe. And while by using 

 any one of these characters we might arbitrarily establish several " genera," 

 they would intergrade so completely in their other characters, we do not 

 think such subdivisions would be either natural or desirable. Accepting 

 Mortensen's view that A. Reynoldsii A. Ag. is a synonym of fenestration 

 Wyv. Thorn., and A. hngispinum Yosh. is identical with A. gracile A. Ag., 

 we still recognize 14 species of Araeosoma. They show great diversity in 

 color, texture of the test, distribution of primary spines, relative number of 

 ambulacral and interambulacral plates, relative width of ambulacra and 

 interambulacra, length of spines, and form of pedicellarioe ; and it is surpris- 

 ingly hard to distinguish them from each other, for not only do their 

 characters reveal more or less individual diversity, but they intermingle 

 most perplexingly in the different forms. We have reached the conclusion 

 that color is often a good character in this genus, and it proves to be of 

 considerable service in distinguishing certain species. The form of the 

 valves of the large tridentate pedicellarioe, which can be easily seen with 

 an ordinary lens, is also a useful character, even if we cannot follow 

 Mortensen in making it generic. The width of the ambulacra and the 

 number of ambulacral plates are valuable, within certain limits, but age 

 differences need to be guarded against, and the same is true of the spines 



