1006 THE VOYAGE OF H.M.S. CHALLENGER. 



Genus 433. LitJwtympanum,^ Haeckel, 1881, Prodromus, p. 447. 



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

 numerous parallel and vertical columellse ; mitral and basal rings of equal size. 



The genus Lithotympanum has the form of a cylindrical drum, the two fundamental 

 faces of which (the mitral and basal gates) are closed by lattice-work. Both horizontal 

 rings are equal, whilst in the preceding Paratympanum the basal gate is larger than 

 the mitral. It may be derived from the latter by equalisation of these two rings, or 

 directly from Dystym'panium by fenestration of the basal ring. 



1. Lithotympanum tuberosum, n. sp. (PL 83, fig. 1). 



Both lattice-plates slightly convex ; the mitral plate with six larger marginal pores and one 

 central pore separated by numerous smaller pores ; the basal plate with seven large pores only (one 

 central and six peripheral). Surface and margin of the plates uneven, with roundish prominent 

 dimpled tubercles. Both rings connected by ten to twelve simple cylindrical vertical columella;. 



Dimensions. — Length of the shell 013, breadth 0"1. 



Habitat. — Western Tropical Pacific, Station 225, depth 4475 fathoms. 



2. Lithotympanum spinosum, n. sp. 



Both lattice-plates flat, with numerous irregular roundish pores of variable size. Surface 

 thorny. Margin with numerous irregular spines, directed upwards on the mitral, downwards on 

 the basal plate. Both rings connected by eight simple cylindrical vertical columella;. 



Dimensions. — Length of the shell 0'12, breadth 0'08. 



Habitat. — Western Tropical Pacific, Station 224, depth 1850 fathoms. 



Subfamily 3. Dystympajstida, Haeckel. 



Definition. — T y m p a n i d a with fenestrated mitral ring and simple open basal ring ; 

 both horizontal rings connected by numerous vertical or divergent columellse. 



Genus 434. Dystympanium,^ n. gen. 



Definition. — Tympanida with two horizontal rings, connected by numerous (six 

 to eight or more) divergent columellse. The upper mitral ring is smaller and latticed, 

 the lower basal ring larger and open. 



^Lithotyrrvpanum—Dvu.moisilex; Ai'^fos, ti//<t«i/o». ^ Dystyvvpaniiim = lliieo'as itum. ; oii;, Tv/^Tra.iiioi'. 



