ASPIDODIADEMA. 93 



most of the interambiilacrum from genital plate to peristome. The oesopli- 

 agus is rather short, and the lower coil of the stomach-intestine is wide 

 and little undulated, though with large radial pouches in all of the ambulacra 

 except the anterior one. The upper coil of the intestine is somewhat nar- 

 rower, and is scarcely undulated at all, so that the entire intestine is remark- 

 ably short. The rectum is held in position by numerous strands of connec- 

 tive tissue which are attached to the abactinal margin of the corona. 



The Genera and Species of AspiDODiADEMATiDiE. 



The better acquainted we become with this characteristically deep-sea 

 group of recent Echini the stronger becomes the conviction that Duncan 

 was right w^hen he separated them as a distinct family from the Diadema- 

 tidge. The group is a very homogeneous one, for while we find it desirable 

 to recognize 10 species, there are only 2 genera, and these are distin- 

 guished solely by the not very important but very constant character of 

 the size of the primary tubercles in the lower half of the ambulacra. It is 

 an interesting fact that essentially the same difference distinguishes Diadema 

 and Echinothrix. The two genera of Aspidodiadematidaj are as follows : 



Large primary tubercles present in ambulacra actinally Aspidodiadema. 



No large primary tubercles present in ambulacra actinally .... Dennatodiadema. 



Aspidodiadema. 



A. Agassiz, 1879. Proc Amer. Acad., XIV., p. 199. 

 Type-species, Aspidodiadema tonsiim A. Agassiz, 1879. Proc. Am. Acad., XIV., p. 199. 



The four species which we consider it desirable to recognize in this genus 

 are very closely allied to each other, but appear to be constantly distinguish- 

 able by the characters given in the table below. One of them, Jacob//i A. Ag., 

 occurs in the West Indian region, in 95-287 fms., but the others occur in 

 the Indo-Pacific region, especially its eastern half, in depths of 100-1700 

 fms. The "Challenger" species, tonsum A. Ag., is known from the East 

 Indies and Japan ; the " Valdivia " species, nicobaricum Dod., from the Nico- 

 bar and Hawaiian Islands; and the ''Siboga" species, meijerei Dtid., from 

 the Kei and the Hawaiian Islands. While nicobaricum conimonlv occurs 

 at depths of over 400 fms., meijerei is usually found in water of less than 

 300 fms. 



