C. E. Beecher — Origin and Significance of Spines. 255 



Bulimina aculeata 9 the surface nodes and granules become 

 developed into spines. In Textularia car incite? and Cristel- 

 laria calcar 9 the carinse are spiniferous. The young of Uvige- 

 rina aculeate? is strongly costate, and later shell growth shows 

 the costse broken up into numerous spines. A related species 

 ( U. aspemda 9 ) has the whole test covered with spinules, which 

 are sometimes arranged in lines, showing derivation from cos- 

 tse. In Truncatulina reticulata* the carina is made up of 

 confluent spines, often discrete along the edge, and sometimes 

 entirely separated. 



37. 



33. 



39. 



Figure 37. — Cyathophycus reticulatus. Ordovioian. £. 

 Figure 38. — Dictyospongia Conradi. Devonian. \. 



Figure 39. — Bydroceras tuberosum. Devonian. £. (Figs. 37. 3S. 39, after Hall.) 

 To illustrate progressive clironogenetic 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- 8 is a turbinate form, 

 with a rectangular mesh of longitudinal and transverse spicular 

 rays (figure 37). At more or less regular intervals, some of the 

 spicules are larger, thus dividing the surface into larger rec- 

 tangular areas. In Dictyospongia prismatica™ from the 

 Devonian, the domination of eight of the longitudinal bundles 

 of spicules has produced a prismatic form. D. Conradi 38 is 

 regularly an eight-sided pyramid or prism when young, but 

 with the growth and elongation of the sponge, it developed 

 slight undulations, then nodes, and later prominent tubercles 

 (figure 38). Ceratodictya annxdata and Hydnoceras nodosum™ 

 show a further specialization in the formation of rings and 

 nodes. Practically the limit to these specializations is attained 

 in Hydnoceras tuberosum™ (figure 39), H. phymedodes and 

 related forms. In IT. tuberosum, the apex representing the 

 young stage or the initial growth is much like Cyathophycus 



