EEPOET ON THE RADIOLAEIA. 1013 



2. Circot^jm'panum octogonium, n. sp. (PI. 94, fig. 17). 



Mitral and basal rings octagonal, the latter one and a half times as broad as the former. Ei'dit 

 columellfe slightly curved, thorny. The mitral ring bears on each corner a short ascending spine, 

 the basal ring a larger, thorny, obliquely descending spine, which has some recurved thorns on 

 the upper edge. All rods more or less thoriiy. 



Dimensions. — Breadth of the mitral ring 0'06, of the basal ring 0"09. 



Habitat. — Central Pacific, Station 271, depth 2425 fathoms. 



3. Clrcotympanum decagonium, n. sp. 



Mitral and basal rings circular, the latter nearly twice as broad as the foi-mer, both armed with 

 scattered irregular thorns. Ten columellie smooth, curved. 



Dimensions. — Breadth of the mitral ring 0'O7, of the basal ring 012. 

 Habitat. — North Pacific, Station 244, depth 2900 fathoms. 



Genus 440. Eiitynvpanium,^ Haeckel, 1881, Prodromus, p. 447. 



Definition. — T y m p a n i d a with two simple equal horizontal rings, connected by 

 numerous (six to eight or more) parallel and vertical columellae. 



The genus Eutympantum contains those Tympanida in which the drum-shaped shell is 

 composed of two simple, parallel and equal horizontal rings, and numerous vertical and 

 parallel rods or columellee connecting them. The number of the latter is at least six or 

 eight. Two opposite of these must be regarded as remaining vertical parts of the sagittal 

 ring, t"wo others as parts of the fi'ontal ring, whilst the other columellse (between the 

 former and the latter) are secondary and intercalated. Therefore Eutympanium may be 

 derived from Tympanidium by reduction and loss of the apical and basal parts of the 

 sagittal ring. 



1. Eittymjmnium musicantum, n. sp. (PI. 83, fig. 2). 



Eight parallel and vertical columellte, simple, straight and smooth. Both horizontal rings equal, 

 circular, each armed with eight to twelve short conical thorns. 

 Dimensions. — Height of the shell 0'09, breadth 0'07. 

 Habitat. — Central Pacific, Station 266, depth 2750 fathoms. 



