THE MODIFICATION OF GENERIC CHARACTERS. 289 



sides of the opposite lamellae. In Zygospira the loop is sim- 

 ple, but arched or forming a double bow-like curve (Figs. 73 

 76). In Dayia there is a simple process from the centre of 

 the saddle running toward the base of the crura (see Fig. 

 8 1 , c). There is added a bifurcated end in Whitfieldia (8 1 , /). 

 In Athyris (81, Ji) the ends of the branches are curved over to 

 partly cover the primary lamellae of the spirals. In Kayseria 

 they are continued along parallel to the lamellae of the spiral 

 coil (8 1, t). This extension is only seen in the late Mesozoic 

 forms. In Meristella (/) the branches of the process recurve 

 and join together, forming on each side a loop, resembling the 

 handles of a pair of scissors. 



In this series of modifications the extreme degree of elab- 

 oration is met with among the Meristellinae, and this subfam- 

 ily was well represented among the Eosilurian faunas. 



Characters of the Brachidium found to be good Distinctive Char- 

 acters of Genera, It has been acknowledged by all the more 

 advanced students of Brachiopods, that the modifications of 

 the brachidium are the most important characters to be found 

 for determining the generic and higher affinities of these in- 

 teresting forms, and great and most painstaking labor has 

 been expended within the past ten or fifteen years in working 

 out the structure of their delicate parts. 



We may interpret this experience of systematists to mean 

 that the various degrees of modification observed in these 

 parts are found to be constant among species which by like- 

 ness in other characters are associated together into groups to 

 form genera. 



Plasticity a Characteristic of their Early Initial Stage. We 

 have already seen by analysis of the characters that almost 

 without exception the plasticity of the characters, and the ex- 

 pression of the widest range of possible differentiation in each 

 particular direction, were characteristics of the early stage in 

 the history of the Helicopegmata. By the beginning of the 

 Neosilurian the expansion of differentiation had reached its 

 extreme in almost every particular. 



Evolution of the Characters of the Brachidium Relatively Rapid. 

 When we consider that we have knowledge of only a few 

 small types of this whole order earlier than the Eosilurian, and 



