ORIGIN AND SIGNIFICANCE OF SPINES 77 



by the writer" to have very pronounced tendencies toward 

 proliferation, and on the interior of the colony these attempts 

 result in the production of mural pores. Most of the species 

 of 3Iichelinia are hemispherical or spherical. M. favosa is 

 inclined to be pyriform in shape, rising above the object of 

 support, and thus presenting a rather large epithecal surface. 

 Manifestly the lower side of the corallum is unfavorably situ- 

 ated for the growth of corallites, and any efforts at prolifera- 

 tion on the part of the peripheral corallites is apt to result in 

 stunted outgrowths. There is here a very close connection 

 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. (Ca-) 



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 be best 

 explained as due to the mechanical restraint of the environ- 

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

 categories of origin as complete as possible. 



The illustrations will be taken chiefly from the Brachiopoda 

 and Trilobita. The recent brachiopod 3Iulilfeldtia 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 

 specimens 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. 

 Specimens growing on a large flat surface have the cardinal 

 extremities angular or sub-mucronate. Similar variations are 



