256 C. K Beecher — Origin and Significance of Sjjines. 



or Dictyospongia. This is followed by a prismatic stage like 

 D. prismatica and J). Conradi, fchen the nodes and tubercles 

 are introduced and further growth produces the typical charac- 

 ters of the species. The tubercles are surmounted by a sharp 

 spine formed at the intersection of two spicular larninsB, 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, alter- 

 nating, elevated and depressed quadrules, the former frequently 



having the superficial layer of spicules extended into a spini- 

 form process, as in P. Dawsoni.™ 



Among corals, there is occasionally some evidence of the 

 external differentiation of structures into spines. The epitheca 

 of the Tetracoralla 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 Cyathaxonia cyno- 

 don 18 and Zap/irentis spinulosa. 18 



Many Crinoids and Asteroids 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 inter- 

 section ; as in Gilbertsocrinus tuber osus, 60 Technocrinus spinu- 

 losus, 69 Actinocrinus lobatus,™ A. pemodosus, Greasier occi- 

 dentalism 0. gigas, Retaster cribrosus, etc. 



The concentric laminse of growth in the Braehiopods are 

 frequently differentiated into spinules ; as in Siphonotreta 

 imguicidata, 31 Schizambon typicalis 31 Spirifer fimbriatus 31 S. 

 pseudolineatus, 31 S. setigerus 31 Cliothyris Royssii 31 etc. Other 

 species show the differentiation of the radii into spines ; as 

 Acanthothyris spinosa™ and A. Doderleini. 1 ' In others the 

 strong concentric laminae passing over radii are often infolded 

 into spines ; as in Atrypa spinosa. 31 



the Mollusca, innumerable examples could be cited 

 clearly the differentiation of various ornamental 

 features into spines. Some of these 

 have already been discussed, but 

 may be referred to again in this con- 

 nection. Thus, an illustration of the 

 passage of concentric laminae into 

 spines is shown in Avicula sterna 36 

 and Anomia acideata 36 (figures 26 

 and 28), and Margaritiphora fim- 

 briata, etc. Many species of Gastro- 

 pods show the same types of differen- 

 tiation. The differentiation of radi- 

 ating lines or ridges into spines is 

 equally common, and is well shown 

 in Spondylus (figures 12, 14, 30), and 

 in Lima squamosus (figure 40). In 



Among 

 showing 



Figure 40. Lima squamosus 

 Natural size. To show differ- 

 entiation of radii into spines. 



