276 STUDIES IN EVOLUTION 



more complicated by two additional lobes. This simple 

 method of increase is further elaborated in the Thecidiidse. 

 In the higher genera, especially among the Terebratulidse, 

 the maximum is reached by means of a median, unpaired, 

 coiled arm, as in Magellania and Terebratulina. 



The development of the different types and varieties of 

 arm structure is presented in the accompanying figures (121- 

 125), which are necessarily somewhat diagrammatic in order 

 to show the features clearly, but the essential structure can 

 be readily verified from consultation of the works cited or 

 from a study of actual specimens. In the case of fossil forms, 

 such as Dielasma, the Atrypidse, and Athyridse, the brach- 

 ial supports have sufficient analogy with the arm structures 

 of Terebratulina and Rhynchonella to warrant their interpreta^ 

 tion as given. Also, the spiral impressions on the valves of 

 Davidsonia, and those occasionally present in Leptcena and 

 Productus, clearly point to the possession of coiled arms by 

 these genera. 



Classification of Brachial Structures. 



From what has already been shown it is seen that the 

 various types of lophophore admit of a simple classification 

 into stages and groups. It is proposed to give to these 

 distinctive names, which may be used with facility in making 

 comparisons and correlations. They may be found useful, 

 also, in designating the kind of brachial complexity attained 

 in any genus the arm structure of which can be determined, 

 thus helping to fix its place in a genetic scale. It should be 

 emphasized, however, that the form and complexity of the 

 cirrated margin of the lophophore can have a taxonomic value 

 only within comparatively narrow limits. This at once be- 

 comes evident when the arms of Lingula, Discinisca, Crania, 

 Rhynchonella, and all the Spiriferacea are considered. Each 

 has spiral arms, which were probably developed through 

 similar changes of form, and yet each is genetically distinct, 



