of Microscopical Organisms. 341 



also materially in their internal structure. Fig. 51, tiie 

 side view of Fig. 50, has only one very large median in- 

 flation, \vith two very small terminal inllations. The cen- 

 tre of the frustule in front view is divided up by transverse 

 bars into nearly equal squares, as in T. musica, E., while 

 in Figs. 46, 47, and 52, which in other respects nearly 

 resemble T. mnsica, we find no such transverse bars. 

 Again, in Fig. 48 there are four horizontal bars, but no 

 vertical. 



It is possible that these should merely be considered as 

 varieties, but they were not so regarded by their discov- 

 erer, and I have concluded to describe them as distinct 

 species. The species represented by Fig. 47 is less coarsely 

 granulated than the varieties of T. mnsica occurring with 

 it. The granules of Fig. 46 agree with those of T. musica. 



POLYMYXUS, Bail., nov. gen. 



Silicious, free, simple, bivalve ; in front view quadran- 

 gular, with undulate ends ; in lateral view circular ; valves 

 composed of curved ridges, appearing in front view like 

 mamillae, but in lateral view tapering to the depressed 

 and stellate centre ; summits of ridges armed with minute 

 spines (?). 



Figs. 55-59. PoLYMYxus coronalis, L. W. B., nov. sp. 

 Frustules large, symmetrical, bivalve ; in front view quad- 

 rangular with mammillated ends ; in lateral view circular ; 

 lateral surfaces of valves elongated into projections which 

 in front view appear like mamillae but in oblique view taper 

 down to the depressed centre ; summits of the elevations 

 terminated by minute spines on the margin of the shell ; 

 valves and median band minutely punctate. 



Hab. Para River, and mouth of Amazon. 



This beautiful species, which makes up the great bulk 

 of the soundings of the Para, seems to be confined almost 

 solely to this one locality. I have detected one specimen 



