DEVELOPMENT OF THE BRACHIOPODA 267 



Preliminary to a study of the stages of growth observed 

 in the different orders, a simple characteristic example of 

 each will be taken to show the limitations of the post- 

 embryonic periods. 



Nepionic Period. — In brachiopods, as in pelecypods, this 

 period represents the growth of the true shell immediately 

 succeeding the embryonic shell or protegulum, and before 

 the appearance of definite specific characters. In general, 

 the nepionic shells of all groups are marked only by fine 

 concentric lines of growth, and are therefore nearly smooth. 

 Sometimes, however, a few radiating stritie or other orna- 

 ments may appear over the nepionic portion, but this is not 

 the prevailing rule. Obolus j^ulcher Matthew shows a can- 

 cellated nepionic-stage and is one of the most striking excep- 

 tional examples. 



Plate XII, figure 1, represents the nepionic stage of Glot- 

 tidia alMda, drawn from the beak of a well-preserved adult. 

 The shell at this period had a short straight hinge (originally 

 the hinge of the protegulum), with lines representing ante- 

 rior and lateral growth, making the outline broadly ovate. 

 It is divided from the succeeding growth of later stages by 

 a strong varix. The form is suggestive of Obolella, and as 

 this is the early form of growth of many of the Lingulidae 

 and allied families, it is here called the Obolella stage. It is 

 not known that otherwise the characters agree with those of 

 Obolella^ but as it is characteristic as well as descriptive the 

 name is used to designate this form of nepionic growth when- 

 ever present. 



The nepionic stage of Orbiculoidea minuta (figure 4) shows 

 a continuance of the straight-hinged condition after the com- 

 pletion of the embryonic shell, with nearly equal incremental 

 lines. As this agrees with the shell of Paterina [= Iphidea^ 

 it is called the Paterina stage. The pedicle emerged freely 

 between the cardinal margins of the valves. It will be 

 shown that both this and the Obolella stage are represented 

 in the nepionic periods of many genera belonging to the 

 Atremata. They may succeed each other in a single species 



