C. E. Beecher — Origin and Significance of Spines. 331 



A number of generally or highly spinose types will now be 

 noted to illustrate the limits of the repetition of spiny struc- 

 tures, the first spines having probably arisen through the 

 operation of some primary cause, and the derived or secondary 

 spines being produced, it is believed, by the law of repetition. 



The Padiolaria have already been frequently mentioned, but 

 as they are the most spiniferous of all classes of animals, and 

 represent the highest degree of spine differentiation attained 

 (figure 51 and Plate I), another brief notice will be of interest. 

 These spines furnish characters of high taxonomic value, 

 although generally speaking they seldom have more than spe- 

 cific importance among other classes. The Echinoidea and 

 Asteroidea must also be noticed in this connection, though 

 from the nature and origin of their spines, they do not con- 

 form to the mode of spine growth in other classes. 



Productus, Productella, Strop halosia, Aulosteges, and 

 Siphonotreta represent highly spinose genera among the 

 Brachiopoda. 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 spi- 

 nosity, 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 ; Echidnoceras, 

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

 (Diodon), Pipe-fish, etc., among the Pisces. The higher ani- 

 mals also furnish examples of extreme spinosity, as in the 

 Horned-Toad (Ph?y?wsoma), the genera of Ceratopsidse, gigan- 

 tic Cretaceous Dinosaurs, 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 offense and defense, as they are often not located in 

 the most advantageous positions ; nor are they differentiated 

 out of any previous ornaments or special structures. In fact, 

 no factor of spine genesis except the one of repetition seems 

 to be sufficient to account for their development. 



VIII. Restraint of environment causing suppression of structures. 



(O,.) 

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

 been brought about by some process of growth or concrescence 



