4. DEVELOPMENT OF SOME SILURIAN 

 BRACHIOPODA * 



(Plates XV-XXII) 



Introduction 



The fossil faunas of rock systems rarely furnish mate- 

 rial for tracing the individual development of any of the 

 contained species. Much will doubtless be done toward 

 ascertaining such development when large collections from 

 suitable localities have been studied with this object in view, 

 and when the number of new species discovered and described 

 each year approaches a minimum. A comparatively full 

 and satisfactory account of the development of the individual 

 organism in several species of trilobites is given in the 

 works of Barrande, Walcott, Ford, and Matthew ; Hyatt, 

 Branco, Mojsisovics, and others have demonstrated the devel- 

 opmental characters of many of the fossil cephalopods, and 

 Verworn has elicited similar facts from certain extinct species 

 of Ostracoda. Further than this but little has been attempted, 

 although the field is a most extensive, important, and invit- 

 ing one. 



As a general rule, the treatment of fossil organisms has 

 rested mainly with geologists having more or less of a zoolo- 

 gical training, and the principal aim has been to present the 

 faunal aspects of each horizon for the purpose of chrono- 

 logical identification. This process has frequently become so 

 involved with the imperfect description of species, that the 

 systematic zoologist or paleontologist is unable to make any 



* Beecher and Clarke. Mem. N. Y. State Mus., I, 1-95, pis. i-viii, 1889. 

 The order of the names of the authors of this paper is without signifii'ance. 

 The work was equally divided and jointly reviewed. 



