ORIGIN AND SIGNIFICANCE OF SPINES 



49 



results in forms consisting only of a composite spine ; as in 

 the legion Acantharia. 



In the Foraminifera there are many instances of the 

 gradual differentiation of carinse, ribs, costse, etc., into spines. 

 In Bulimina aculeata 9 the surface nodes and granules become 

 developed into spines. In Textularia carinata 9 and Cristel- 

 laria calcar Q the carinae are spiniferous. The young of 

 Uvigerina aculeata 9 is strongly costate, and later shell growth 

 shows the costse broken up into numerous spines. A re- 

 lated species ( U. asperula 9 ) has the whole test covered with 

 spinules, which are sometimes arranged in lines, showing 

 derivation from costse. In Truncatulina reticulata 9 the 

 carina is made up of confluent spines, often discrete along 

 the edge, and sometimes entirely separated. 



37 



38 



39 



FIGURE 37. Cyaihophycus reticulatus. Ordovician. . 



FIGURE 38. Dictyospongia Conradi. Devonian. \. 



FIGURE 39. Hydnoceras tuberosum. Devonian. . (Figures 37, 38, 39, 

 after Hall.) 



To illustrate progressive chronogenetic and ontogenetic differentiation in a 

 family of hexactinellid sponges. 



The hexactinellid sponges belonging to the family Dictyo- 

 spongidse show some very clear instances of the progressive 

 differentiation of ornament in time and in ontogeny. The 

 Ordovician Cyathophycus reticulatus 28 is a turbinate form, 

 with a rectangular mesh of longitudinal and transverse 

 spicular rays (figure 37). At more or less regular intervals 



