C. E. Beecher — Origin and Significance of Spines. 337 



between restraint of environment and deficiency of growth 

 force. If the whole corallum is taken into consideration, the 

 restraint of the environment may be taken as preventing the 

 growth of corallites on the lower side. If one of these single 

 stunted corallites is considered, it may be said that the defi- 

 ciency of growth force through lack of nutrition caused its 

 suppression. 



IX. Mechanical restraint. (C Q .) • 



Among the factors of spine genesis, mechanical restraint is 

 probably of the least importance. It can only rarely happen 

 that an organism is forced to grow a spine contrary to the 

 natural tendencies of normal development. Yet as there are 

 occasional types of spiniform structures which can best be 

 explained as due to the mechanical restraint of the environ- 

 ment, it is necessary to notice them in order to make the cate- 

 gories of origin as complete as possible. 



The illustrations will be taken chiefly from the Brachiopoda 

 and Trilobita. The recent Brachiopod Muhlfeldtia truncata 

 is semi-elliptical in outline, and has a very short stout pedicle 

 which holds the shell so closely to the object of support that 

 the beak is truncated from abrasion and resorption. In speci- 

 mens attached to a small branch of a coral, thus allowing the 

 cardinal extremities of the shell to project beyond the object 

 of support, the ends of the hinge are generally rounded. Speci- 

 mens growing on a large flat surface have the cardinal extremi- 

 ties angular or submucronate. Similar variations are to be 

 observed in other living species of Brachiopods [Cistella, some 

 Dallina, etc.). Some of the extinct genera show more highly 

 developed cardinal extremities which are often very character- 

 istic of certain species, though considerable variation is found 

 to exist. It is evident that these elongated hinge lines have 

 arisen from the mechanical necessities of a functional hinge, 

 and their greater or less extent is also to a degree dependent 

 upon the nature of the object of support which furnishes a 

 stimulus to the growing ends of the hinge. A marked exam- 

 ple is shown in Spirifer mucronatus, QQ 

 with the cardinal angles extended into 

 spiniform processes (figure 60). Similar 

 features are presented by many other 

 species of Spirifer, Orthis, Zeptcena, 



btroplieodonta, etc. _ _ of Spirifer mucronatus, De- 



In the Trilobites, the pygidilim, 01' vonian. Showing spiniform 



abdominal portion, consists of a number ^n and^ke ) X f ' (After 

 of consolidated segments, and the seg- 

 ments of the thorax are successively added in front of this tail 

 piece. The first thoracic segment is therefore formed between 



