INTRODUCTION 9 



limited number of species, and because they undergo no important modi- 

 fication in their subsequent growth, and merely represent the apical portion 

 of mature individuals. Among the crinoids, the modifications of form and 

 structure from the embryo state to maturity are more profound and essen- 

 tial, although the material is not sufficiently complete to furnish any very 

 important results. 



It is necessary to state that nearly all our observations on the develop- 

 ment of the brachiopoda are based upon the study of the material derived 

 from a single locality, and some of the minor deductions may not apply, in 

 every case, to the individuals of the same species found in other regions. 

 The writers have also refrained, except when essential to the proper expo- 

 sition of a species, from entering into details of synonymy or generic contro- 

 versy as to the correct reference of the species. This course is considered 

 advisable, from a desire not to introduce any discussions alien to the 

 descriptions of the developmental changes in these organisms. Aside from 

 this, it is believed that a number of important facts are here added to the 

 knowledge of the brachiopoda, and that many of them will be found to be 

 of general application. The investigation has also resulted in elucidating 

 several obscure and anomalous features of the shell and of the cardinal 

 area, which appear in their proper place in the description of the spe- 

 cies and in the general summary. 



The following list includes all the species of brachiopoda which, up to 

 this time, have been described from the shales at Waldron, Indiana, and 

 comprises forty-two species and varieties, ascribed to twenty-four genera. 

 It also shows whether material has been obtained which furnishes data 

 for tracing the developmental changes. 



The majority of the species which have afforded no young specimens 

 are rare forms even in their adult state. Among the actually abundant 

 species of which there are no means accessible of tracing the life-history, 

 Rhynchonella stricklandi is a noticeable example, and it is really the only 



