50 STUDIES IN EVOLUTION 



some of the spicules are larger, thus dividing the surface 

 into larger rectangular areas. In Dictyospongia prismatica 28 

 from the Devonian, the domination of eight of the longi- 

 tudinal bundles of spicules has produced a prismatic form. 

 D. Oonradi 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 annulata 

 and Hydnoceras nodosum 28 show a further specialization in 

 the formation of rings and nodes. Practically the limit to 

 these specializations is attained in Hydnoceras tuberosum 28 

 (figure 39), H. phymatodes, and related forms. In H. tubero- 

 sum the apex representing the young stage or the initial 

 growth is much like Cyathophycus or Dictyospongia. This is 

 followed by a prismatic stage like D. prismatica and D. Con- 

 radi-, then the nodes and tubercles are introduced and further 

 growth produces the typical characters of the species. The 

 tubercles are surmounted by a sharp spine formed at the 

 intersection of two spicular laminse, one concentric and one 

 longitudinal. 



Another type of surface specialization is shown in the 

 genus Physospongia from the Keokuk group of the Lower 

 Carboniferous. In this genus there are bands of regular, 

 alternating, elevated, and depressed quadrules, the former 

 frequently having the superficial layer of spicules extended 

 into a spiniform process; as in P. Dawsoni. 28 



Among corals there is occasionally some evidence of the 

 external differentiation of structures into spines. The epi- 

 theca of the Tetracorolla frequently shows, by means of low 

 lines or low ridges, the number and direction of the septa, 

 and in some of the later species these external septal lines 

 are ornamented with rows of short spines or spinules; as in 

 CyatJiaxonia cynodon 18 and Zaphrentis spi?iulosa. 18 



Many Crinoidea and Asteroidea show the development of 

 tubercles into spines, and the surface sculpture is often 

 made up of ridges which bear strong spines at the points of 

 intersection; as in Grilbertsocrinus tulerosus^ Technocrinus 



