ISO THE RADIOLARIA 



E. Mid-sized forms, without primary shell. Secondary shell with 

 four arms. Atlanticella. (Fig. 28.) Borgert (21). 



FAMILY 3. CASTANELLIDAE. Primary shell two - layered and com- 

 posed of (1) extremely delicate tangential siliceous needles; (2) the 

 two conjoined limiting membranes of the two layers, united by (3) a 

 porcellanous impregnation. Selected form : Castanidium apsteini, 

 bipolar (36). 



FAMILY 4. CIRCOPORIDAE. Shell composed as in Family 3, but 

 spherical, polyhedral, or multipolar (36). 



FAMILY 5. TDSCARORIDAE (Fig. 30). Shell rarely spherical, gener- 

 ally monaxonic. Nucleus elongated with sigmoid chromatin band. 

 (Borgert [2 la].) 



FIG. 30. 



Tnscaroridae. A, Tuscarusa globosa, Borgert, showing the poristomial hollow spines ; 

 the rest are broken off. x 39. B, Tuscarora nationalis, Borgert, showing the two central 

 capsules in the shell. Each capsule contains a bent nucleus, x 24. (After Borgert.) 



ORDER 4. Phaeoconchia, H. 



Central portion of the skeleton in the form of two valves, free or 

 hinged together. 



FAMILY 1. CONCH ARID A E, H. With thick valves, which are devoid 

 of an apical cupola and of radial tubes. Equatorial and southern 

 forms. 



FAMILY 2. COELODENDRIDAE. With extremely thin valves, each of 

 which bears a cupola and tubular processes. Coelodendron ramosissimum, 

 Faroe Channel and cosmopolitan. 



FAMILY 3. COELOGRAPHIDAE. Each cupola provided with a hollow 

 process (rhizocanna), which communicates with the cupola by a paired 

 or unpaired frenulum. Radial tubes strongly developed, sometimes 

 forming an outer bivalved mantle. The largest and most complex 



