126 



PROTOZOA. 



The former has a comparatively large fusiform shell, consisting of 

 many layers of chambers rolled up spirally round an elongated axis, 

 the last series opening by a row of pores. It dates from the Cre- 

 taceous period, and has largely contributed to the formation of 

 various of the Tertiary limestones. The latter is coin-shaped, some- 

 times more than half an inch in diameter, and very complex as 

 regards the arrangement of its chambers. The genus is especially 

 abundant in the Eocene Tertiary, but it dates from the Lias, and 

 occurs plentifully in recent seas. 



The next family of the Foraminifera is that of the Astrorhizidce, 

 comprising a number of interesting types, sometimes of considerable 

 size, in which the test is composed of sand-grains or other foreign 

 particles, generally united by a cementing basis, but sometimes 

 slightly or not at all consolidated. The test may be monothalamous 

 or polythalamous, but in the latter case genuine internal septa do 

 not exist. The shell is often branched or radiate, and may attain a 

 considerable size. The type-genus of this family is Astrorhiza itself 

 (fig. 19, a), which is not known to occur in the fossil condition, its 

 test, though of considerable thickness, having its component grains 

 very imperfectly cemented together. A much firmer and more com- 

 pact shell is possessed by the genus Saccammina, which merits special 



mention as being the only Foramini- 

 fer which in Britain can be said to 

 actually form a limestone. It con- 

 sists of free spherical, pyriform, or 

 fusiform chambers (fig. 30), some- 

 times separate, sometimes united 

 end to end in twos or threes, with 

 thick, internally labyrinthic walls. 

 The central chamber communicates 

 with the exterior by a single aper- 

 ture, and the average length of the 

 chambers of the British Carbonifer- 

 ous species (Saccatnmina Carteri, 

 Brady) is as much as i-8th inch. 

 It forms beds of limestone in the 

 Carboniferous of the South of Scot- 

 land and North of England ; but 



B 



c W> o <> 



Fig. 30. — A, A slice of limestone with Sac- 

 cajujuina Carteri, enlarged five diameters, 

 from the Carboniferous Limestone of Elf- 

 hills, Northumberland. (After h. b. Brady.) the genus is not known to occur 



B, Spheres of the same, of the natural size, „„„:,_ *-; , \'\ , TT .~ ,-^^^t- ,"*. ,"^ </u~ t> j_ 



exhibiting variations. a g am till we meet it in the Post- 



Pliocene, and, in a living state, in 

 the North Sea. The genus is, however, of a much higher antiquity 

 than the Carboniferous, since it is found to be largely represented 

 in the Ordovician limestones of Ayrshire. 



In a number of very interesting forms of the Ast?-orhizidce, the test 



