DEVELOPMENT OF SOME SILURIAN BRACHIOPODA 313 



this region, and by the rapid sedimentation of the shales. 

 The richness of the material is shown by the great profusion 

 of specimens representing the sponges, corals, crinoids, Bry- 

 ozoa, brachiopods, gastropods, annelids, and crustaceans, com- 

 prising altogether about one hundred and fifty species. The 

 lamellibranchs and cephalopods were also doubtless abundant ; 

 but the conditions existing for the preservation of their 

 remains were not favorable, probably on account of the com- 

 position of their shells, and but sixteen species have been 

 noted. That the fauna was protected from excessive storms 

 and the action of sea currents, is evinced by the usual perfec- 

 tion of the fossils. Some of the crinoids are unbroken and 

 remain attached by their roots, retaining their arms in place ; 

 also large colonies of delicate branching corals and Bryozoa 

 still preserve their unity. The specimens were rapidly buried 

 in the soft calcareous mud, and show none of the eroding or 

 disintegrating action of the water, such as would have been 

 produced had they lain for any considerable period unpro- 

 tected on the sea-bottom. It is f true that many specimens are 

 incrusted with Bryozoa, annelids, Cranias, and other fixed and 

 incrusting forms, but the majority of these seem to have 

 flourished during the life of their hosts. 



Besides the embryonic Brachiopoda occurring in these 

 shales, there are other classes represented by immature forms, 

 notably the Gastropoda and Crinoidea. These, with the 

 Brachiopoda, embrace almost all the young forms found. The 

 small gastropods are of little interest, on account of the 

 limited number of species, and because they undergo no im- 

 portant modification 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 essential, 

 although the material is not sufficiently complete to furnish 

 any very important results. 



It is necessary to state that nearly all these observations on 

 the development of the Brachiopoda are based upon the study 

 of the material derived from a single locality, and some 



