ORIGIN AND SIGNIFICANCE OF SPINES 25 



to unite, and this tendency to the formation of tubules is 

 apparently carried no further at this period. More extreme 

 results were attained by the Atrypa aspera of the Hamilton 

 shales, or possibly by its migrated ancestor, during the period 

 of time represented by tlie deposition of the Lower Helder- 

 Ijcrg, Oriskany, and Upper Helderberg sediments. At all 

 events, the Atrijpa spinosa of the Hamilton shales is but an 

 ^1. axpcrn with the lamellae enfolded into tubular spines. 

 Intermediate stages connecting these different phases are not 

 present in this fauna. . . . This spinose form is continued 

 into the Chemung faunas (A. hystrix), with some modifi- 

 cation of expression, the spines being few and long, and the 

 plication of the surface veiy coarse and quite simple; the 

 shell in its decline thus representing a decided return to 

 the primitive type of structure." H. S. Williams ^^ has classi- 

 fied the variations in the stock of A. reticularis as to whether 

 differentiation in the number of plications is increased or 

 retarded, and concludes that the extremes are most strongly 

 expressed at the close of the life-period of the race. The 

 numerously plicated type represents the accelerated phase of 

 the multiplication of radii, while A. hystrix, with its few and 

 coarse radii, represents the retardation or suppression of this 

 tendency. 



The only great group of animals receiving its name from 

 its characteristically spinose surface is the Echinoderraata, or 

 the spiny-skinned animals ; yet it is extremely doubtful 

 whether this name would have been used had the first studies 

 of the group been based upon the Paleozoic representatives, 

 especially the pre-Devonian species. The early Sea-lilies 

 (Crinoidea), Cystideans (Cystoidea), Blastoids (Blastoidea), 

 and Star-fishes (Asteroidea) had smooth or nearly smooth 

 integuments. In its early genera, even the most typically 

 spiny class of the whole sul>kingdom, the Echinoidea (Sea- 

 urchins), had very minute and insignificant spines. It is only 

 in the late Devonian and in the Carboniferous that truly 

 spiny forms of Crinoids, Star-fishes, and Sea-urchins are 

 found. 



