DEVELOPMENT OF THE BRACHIOPODA 237 



ontogeny of one of the Permian species. Strophahnia GoJd- 

 fussi, in early neanic stages, has a hinge-line about equal 

 to the width of the shell, but in mature individuals it is 

 usually less than one-half the width. This reduction of the 

 hinge and ostrean form of growth are in accordance with the 

 deductions and observations made upon the Oyster and its 

 allies by Jackson, and the mechanical principles are evidently 

 the same in both cases. 



One of the most conspicuous examples of a difference in 

 the form of the valves is shown in the abnormal genus Pro- 

 bosciclelhi. In early neanic stages it resembles an ordinary 

 Productus. Afterward, probably from burrowing in the 

 mud, the ventral valve becomes exti'avagantly developed 

 anteriorly into a calcareous tube. This is accomplished by 

 the excessive growth of the anterior and lateral margins. 

 Then an infolding takes place until the lateral edges unite, 

 after which the tube is built up by concenti-ic increment 

 around the free end. The resemblance of ProhoHciddla to 

 Asperf/iUinn is quite marked, except that, in the latter genus, 

 the tube is formed from the growth and union of two valves 

 instead of one. 



From the morphological differences of the ventral and 

 dorsal valves it will be seen that the highest modifications 

 occur in the former; while the variations in the latter are 

 expressed mainly as adaptive reactions or accommodations to 

 these changes. The explanation of the fact that greater 

 alteration takes place in the ventral valve evidently lies not 

 in the greater plasticity of this member, but in its more 

 highly specialized and differentiated external form, and 

 mainly in its being the lower and attached valve. 



No account is taken here of the crura, loops, and spires of 

 the brachial valve, so characteristic and important in many 

 families and genera. These are evidently processes devel- 

 oped by the internal requirements of the animal, and are not 

 affected by the environment. Therefore they are internal 

 calcified organs independent of the form or manner of growth 

 of the external covering. This is shown by the fact that in 



