52 THE MICROSCOPE. Apr., 



posphaera. Or there may be two spheres inside like fig. 

 3. Then it is div. C, either Rodosphsera or Thecosphsera, 

 and if the two inner spheres are close to the centre it is 

 the latter. If on the contrary one inner sphere is close 

 to the outer shell it is the former. Possibly you may 

 detect three spheres inside like figure 4, or even four 

 spheres. In the first case it is Cromyosphsera, in the 

 second, Caryosphsera. 



But suppose it has spines, what then? Why mark the 

 number and how they are placed. Are there two, 

 opposite, like fig. 6? Earn. Stylosphserida. Four, for- 

 ming a cross, like fig. 7 ? Fam. Staurosphserida. Six, 

 opposite in pairs, like fig. 8 ? Fam. Cubosphserida. 

 Eight to twelve or more like fig. 9 ? Astrosphaerida. 

 Take one more look to see whether the sphere in each 

 case is single or has other spheres inside and the rest of 

 the Key will explain itself, the only genus needing any 

 note being Saturnalis, which looks like fig. 6 with a 

 circle drawn around it so that the tips of the spines 

 would just touch the circumference. 



However, your form may not be a true sphere, a sec- 

 tion of which would be like fig. 1, but ellipsoidal, a 

 section of which would be like fig. 10. If so it belongs 

 to the Order Prunoidea. Is the margin entire, like fig. 

 10, or has it a constriction, like fig. 12, forming two 

 chambers, or several constrictions, forming four or more 

 chambers like fig. 13 ? Then it belongs respectively to 

 div. A, or B, or C, of that part of the Key which treats 

 of ellipsoidal shells. If A, is it hollow like fig. 10, and 

 is the surface latticed or covered w4th pores .^ Fam. 

 Ellipsida. And the genus will depend on the absence 

 or presence of spines and the disposition of them. Or 

 has it an inner shell like fig. 11 ? Pruppulida, and the 

 Key will readily give the genus, the only explanation 

 needed being that the medullary shell is the inner as dis- 

 tinguished from the outer which is termed the cortical 



