180 THE AMERICAN MONTHLY [August, 



tingiiish these features of the central capsule, there are marks in 

 the skeleton which correspond, namely, the presence or absence 

 of a large mouth or opening. For when the inner capsule has 

 pores all over it the outer skeleton has also, whereas if there is 

 only one perforated area in the capsule tliere is, as a rule, a wide 

 opening in the skeleton. Furthermore, the spherical capsule of 

 the first group produces round or disk-like skeletons, the cone- 

 shaped capsule of the second group, cone-shaped skeletons, or 

 modifications of the cone. 



If your Radiolarian skeleton then has no large mouth or open- 

 ing it belongs to the first group, which contains these 20 of the 56 

 genera : 



Cenosphcvra^ EthmospJnvra^ Stylosphcvra^ Spoiigosphivra^ 

 Haliomma^ LUhocyclia^ Stylocyclia^ Hymenactura^Astractiira^ 

 Stauractura^ Pentactura^ Echinactura^ Porodisciis^ Pei-ich- 

 lamydium^ Stylodictya^ Hymejiiastrum^ Histlastrnm^ Stepha- 

 nastr7i7n^ Phacodiscus^ Pei-iphcEfia. 



Of these, again, some are spherical and others convex or fiat. 

 If your form is a sphere or very convex, it is one of the first five. 

 And these are the distinguishing characteristics ; Spongosphccra 

 has a spongy skeleton. If there are no radial spines on the sur- 

 face of the shell it is Ccnosp/iccra or Ethmosphivra^ -AwtS. pores 

 prolonged into tubules will make it the latter. If there are two 

 main spines opposite in one axis it is Stylosphccra. This genus, 

 however, is still further divided by Haeckel, according as the spines 

 are equal or unequal or curved or bent, but Ehrenberg classes 

 them all under Stylosphcera. If there are S to 12 or more radial 

 spines it is Haliotmna. 



If your form is slightly convex it is one of the next 9 genera. 

 If the shell is lens-shaped and there are no chambered girdles, it 

 is either Phacodiscus or PeripJuvna^ and u hyaline equatorial 

 girdle will make it the latter. 



If the disk has rings divided by radial beams into chambers it 

 is Lithocyclia., Stylocyclia^ Hymenactura, or oneof the four next 

 and allied genera. LitJiocyclia has no radial spines, .SVi'/ocvc/Za 

 2 opposite spines. The others have from 2 to 5 hollow radial 

 chambered arms. Hyjnenactura has 3, Astractura and Staurac- 

 ttira have 4, Pentactura and Echinacttira ^. Stauractura and 

 Echinactura again are marked by the presence of a connecting 

 structure between the arms. 



LIST OF FIGURES IN THE FRONTISPIFCE. 



Fig. I. Stylosphsera Carduus. Fig 7. Lithornithium foveolatum. 



2. Hymeniastrum (Hisiiastrum) terna- 8. Lilhocircus (Stephanolilhis) spines- 



rium. cens. 



3. Stylocyclia dimidiata. 9. Stauractura (Astromma) Aristotelis. 



4. Sethampora (Eucyrtidium) Mongol- 10. Spongosphjeia rhabdostyla. 



fieri. II. Halicalyptra Galea. 



5. Cornutella ampliata. 12. Pterocodon Campana. 



6. Eucyrtidium Picus. 



(^To be C07ititiued.) 



