ORIGIN AND SIGNIFICANCE OF SPINES 



49 



results iu forms consisting only of a composite spiue ; as in 

 the legion Acantharia. 



In the Foraminifera there are many instances of the 

 gradual differentiation of carinte, ribs, costae, etc., into spines. 

 In Bulimlna aculeata ^ the surface nodes and granules become 

 developed into spines. In Tcxtularia carinata^ and Cristel- 

 laria calcar^ the carinre are spiniferous. The young of 

 Uvigerina aculeata^ is strongly costate, and later shell growth 

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

 lated species (^U. asperula^y has the whole test covered with 

 spinules, which are sometimes arranged in lines, showing 

 derivation from costse. In Truncatulina reticulata ^ the 

 carina is made up of confluent spines, often discrete along 

 the edge, and sometimes entirely separated. 



37 



38 



39 



Figure 37. — Ci/athophycus reticulatus. Ordovician. |. 



Figure 38. — Dlctyosponrjia 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 Oyathophyaus reticulatus''^ is a turbinate form, 

 with a rectangular mesh of longitudinal and transverse 

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



4 



