ORIGIN AND SIGNIFICANCE OF SPINES 



49 



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

 the legion Acantharia. 



In the Forauiinifera there are many instances of the 

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

 In Bulimina aculeata ^ the surface nodes and granules become 

 developed into spines. In Teitidaria carinata^ and Cristel- 

 laria ealcar^ the carince are spiniferous. The young of 

 Uvigerina aculeata ^ is strongly costate, and later shell growth 

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

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

 spiuules, 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 retlcu/atus. Ordovician. ^. 



Figure 38. — Dictyospongia Conradi. Devonian. \. 



Figure 39. — Ilijdnoceras tuberosum. Devonian. J. (Figures 37, 38, 39, 

 after Hall.) 



To illustrate progressive chronogenetic and ontogenetic differentiation in a 

 famiJy of hexactiuellid sponges. 



The hexactiuellid sponges belonging to the famih'- Dictyo- 

 spongidae show some very clear instances of the progressive 

 differentiation of ornament in time and in ontogeny. The 

 Ordovician Ct/atJiojjJii/ciis reticulatus^^ is a turbinate form, 

 with a rectangular mesh of longitudinal and transverse 

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



