PROTOZOA. 



of each lamina as minute pores. Species of the genus occur 

 in both Lower and Upper Silurian strata, and in the Devonian 

 series, apparently not completely disappearing till towards the 

 close of the Triassic period. 



Receptaculites (fig. 20), in its most perfect condition, is 

 described by Mr Billings as being " of a discoid, cylindrical, 



ovate, or globular shape, 

 hollow within, and usually, 

 if not always, with an 

 aperture in the upper side. 

 In or near the centre of 

 the lower side there is 

 generally to be seen a 

 small rounded protuber- 

 ance, indicating, most pro- 

 bably, the position of the 

 primitive cell or nucleus 

 from which the animal 

 commenced its growth. 

 . . . The body-wall is of 



Fig. 2 o.-piagram of the structure of Recep- * SOmewhat Complex StrUC- 



taculites, as it would be shown by a vertical sec- ture. It Consists of three 

 tion of a perfect specimen, a The aperture at the 1 



summit ; b The inner integument ; c The outer parts an external anQ ail 



integument ; The usual position of the nucleus j internal integument, and, 

 v Ihe great internal cayity. The unshaded . ' 



bands running from the outer to the inner integu- between these, a peculiar 



ment represent the tubes. (After Billings.) tubular Or Spicular skele- 



ton." The inner and outer integuments are composed of 

 numerous rhomboidal calcareous plates, closely fitting together, 

 and arranged in peculiar curved rows, giving fragments of the 

 fossil very much the appearance of the engine-turned case of a 

 watch. The inner integument differs from the outer in being 

 pierced by numerous small apertures, which open into the 

 central cavity, an aperture being placed at every junction of 

 four plates. Lastly, the inner and outer integuments are con- 

 nected together by a number of small straight cylindrical tubes 

 or hollow spicula. The late Mr Salter regarded Receptaculites 

 as belonging to the Fbraminifera, and finding its nearest living 

 ally in Orbitolites. Mr Billings, however, points out that it 

 has some curious points of resemblance to the little seed-like 

 " gemmule " of the Fresh-water Sponge \ and he regards it as 

 being on the whole a Sponge, having relationships with the 

 Foraminifera. Receptaculites occurs in both Lower and Upper 

 Silurian strata, as does the nearly-allied or identical genus 

 Ischadites. 



