THE PORIFERA AND PROTOZOA 323 



Recent; Webbina (Vitriwebbina), an irregular row of 

 elliptical chambers, encrusting shells (Jur.-Rec.). 



2. Textularidae, with a zig - zag alternation of 

 chambers. Chief genus : Textularia (Camb.-Rec.). 



3. Globigerinidse, with chambers globose, usually in 

 a somewhat irregular spiral, perforations coarse. Chief 

 genus : Globigerina (Camb.-Rec.), a most abundant 

 pelagic form at the present day. Associated with it is 

 the perfectly spherical Orbulina, of which most specimens 

 when broken open are found quite hollow, but occa- 

 sionally a Globigerina is found inside : this suggests that 

 Orbulina is the final stage of Globigerina, the majority of 

 specimens being megalospheric and omitting the Glo- 

 bigerina-stage in ontogeny. 



4. Rotalidas, with chambers coiled in an asymmetric 

 spiral, all the whorls being visible on one face, and only 

 the last whorl on the other. In this family first appears 

 the supplemental skeleton, an outer calcareous layer plastered 

 over the surface of the ordinary perforate wall, pene- 

 trating between the two folds of the latter which here 

 form the septa, and sometimes forming a surface orna- 

 ment (even long spines in some) which quite obscures the 

 arrangement of the chambers. 



Chief genera : Discorbina (Cret.-Rec.) ; Planovbulina 

 (Carb.-Rec.) ; Truncatit,lina(Ca.rb.-Rec.);Anomalina(Cret.- 

 Rec.) ; Pulvinulina (Jur.-Rec.), with the form described 

 above, but differing in the relative convexity of the two 

 faces, without supplemental skeleton ; Rotalia (Jur.-Rec.), 

 with supplemental skeleton, but recognizably spiral ; 

 Calcarina, discoidal with peripheral spines, spiral inter- 

 nally (Cret.-Rec., chiefly in Maestrichtian) ; Tinoporus, 

 similar shape but chambers irregularly arranged, and 

 surface with pattern of tubercles (Eoc.-Rec., abundant 

 on modern coral-reefs). 



