THE RADIOLAR1A 



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. TOSCARORIDAE (Fig. 30). Shell rarely spherical, gener- 

 ally monaxonic. Nucleus elongated with sigmoid chromatin band. 

 (Borgert [2 la].) 



FIG. 30. 



Tuscaroridae. A, Tuscarusa globosa, Borgert, showing the peristomial 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. CONCHARIDAE, 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. Goelodendron 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 



