C. E. Beechev — Origin and Significance of Spines. 19 



gradation between the extremes, which, therefore, cannot 

 be considered as definite permanent varieties. There are, 

 however, associated forms that have received distinctive spe- 

 cific names, which do not shade into each other. During the 

 early and middle Devonian, certain of these variations in the 

 main stock of A. reticularis became more fixed, and at the 

 time of the Hamilton sediments in New York, there are two 

 forms known as A. reticularis and A. aspera, which ap- 

 parently do not pass into each other. As time went on, these 

 two types became more specialized and the divergence cor- 

 respondingly increased, until in the Upper Devonian, in the 

 Chemung sediments, there is a large many-plicated A. reticu- 

 laris, as well as a form with very few plications and long mar- 

 ginal spines, A. hystrix. Hall and Clarke 31 thus summarize the 

 stages leading to the formation of the spinose forms : "In the 

 variant of Atrypa reticularis, occurring in the Niagara fauna 

 at Waldron, Indiana, the free concentric lamellae frequently 

 show a tendency to fold inward at the summit of the principal 

 plications. The infolded edges fail 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 

 the deposition of the Lower Helderberg, Oriskany, and Upper 

 Helderberg sediments. At all events, the Atrypa spmosa of 

 the Hamilton shales is but an A. aspera 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 modification of expression, the spines 

 being few and long, and the plication of the surface very 

 coarse and quite simple ; the shell in its decline thus represent- 

 ing a decided return to the primitive type of structure." H. 

 S. Williams 72 has classified 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 Echinodermata, 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, 



