70 STUDIES IN EVOLUTION 



notreta represent highly spinose genera among the Brachi- 

 opoda. Strophalosia is a form in which the ventral valve is 

 cemented to some object. "Whenever the valve rises well 

 above the object of support, the spines are free like those 

 frequently present on the dorsal valve ; otherwise the spines 

 extend root-like along the supporting surface (figures 52, 53). 



Aulosteges presents a still further tendency to complete 

 spinosity, for not only are both valves covered with spines, 

 but the deltidium also. 



Spondylus (figure 30) and Murex are well-known types of 

 very spiny forms of Mollusca. Acidaspis, Terataspis, etc., 

 hold the same place among the Trilobita; Uchidnoceras, 

 Lithodes, etc., among the Decapoda; and the Spiny Box-fish 

 QBiodon), Pipe-fish, etc., among the Pisces. The higher 

 animals also furnish examples of extreme spinosity; as in 

 the Horned Toad (^Phrynosoma), the genera of Ceratop- 

 sidee, gigantic Cretaceous Dinosauria, and the Echidna and 

 Porcupine. 



All these forms present numerous spines, some of which 

 cannot be explained as having arisen directly from external 

 stimuli, for they are in comparatively well-protected regions 

 out of the way of external stimuli. Neither can all of them 

 serve for offence and defence, as they are often not located 

 in the most advantageous positions; nor are they differen- 

 tiated out of any previous ornaments or special structures. 

 In fact, no factor of spine genesis except the one of repetition 

 seems to be sufQcient to account for their development. 



VIII. Restraint of environment causing suppression of 

 structures. (Ci.) 



The previous categories of spine production (I-VII) have 

 been brought about by some process of growth or concres- 

 cence through external and internal agencies. There still 

 remains for discussion the formation of spines by processes of 

 decrescence caused by extrinsic restraint (C) or by intrinsic 

 deficiency of growth power (D). The lack of vitality or 



