DEVELOPMENT OF THE JIRACIIIOI'ODA 235 



Chtella the two valves are more strongly specialized, while 

 in JJincinisca, Theddium, and Crania they are quite unlike. 



Two important organic characters accompany and partake 

 of a similar amount of variation; (a) the length and direc- 

 tion of the pedicle, and (?>) the position and structure of the 

 pL'dicle-opening. Lirujula with a long, fleshy, mohile pedicle 

 receives uniformly disposed axil impacts on the valves, and 

 therefore, with equal physiological reactions, equality in size 

 and form is produced. Terebratulma and most of the other 

 terebratuloids and rhynchonelloids have a shorter and less 

 flexible pedicle. As a whole the motions of the animal are 

 more restricted; the pedicle-opening is confined mainly to 

 one valve; the valves, consequently, are differently related 

 to the environment, and express this difference in their dis- 

 similarity. In these examples, also, the inclination of the 

 pedicle to the longitudinal axis, or of the shell to the surface 

 of support, agrees, ^;arz passu, with the amount of unlikeness 

 in the valves, except when the pedicle is so shortened as to 

 interfere with their free movement. To this inclination is 

 probably due the difference in the action of the forces from 

 without. 



Normally, in Lingula, the pedicle is in direct linear con- 

 tinuation with the axis of the shell. Terehratulina and 

 M(i<j Mania are inclined at an angle of 40° to the surface of 

 support, but in Oistella and MiMfeldtia this is increased to 

 about 70°. In the latter genera, although the position of 

 the axis is nearly vertical, the shortening of the pedicle pre- 

 cludes more than a slight elevation and rotation of the organ- 

 ism. The more the pedicle-opening is confined to one valve 

 the greater is the difference between both. 



Passing to Discinisca, the pedicle is found to be at right 

 angles to the longitudinal axis, and the valves become strictly 

 an upper and a lower. The lower rests upon the object of 

 support, and the animal is capable of raising and rotating it 

 only to a slight degree. Under such circumstances the 

 lower valve is wholly different in its relations to the environ- 

 ment, and, naturally, it expresses the greatest dissimilarity 



