THE ACQUIREMENT OF CHARACTERS ILLUSTRATED. 



thus shown in their strong historical contrast, the former 

 (/-/' ', Fig. 58) culminating its generic evolution in the Ordo- 

 vician, while the Arthropomata (a-a') culminates in the 

 Silurian, but continues to differentiate until the middle of 

 the Mesozoic ; and the several distinct lines of differentiation 

 are expressed by the curves for the several suborders, which 

 are recognized in the classification of Schuchert.* 



This classification recognizes the Brachiopoda as a class, 

 and Arthropomata and Lyopomata as subclasses. The 

 Arthropomata (a-a') are divided into two orders, (i) the Pro- 

 tremata (/-/') (Beecher), including two suborders, viz., 

 Trullacea (b-b'\ and Thecacea (c-c f ) ; and a second order, 

 Telotremata (t-t r ), with the suborders Rostracea, Helicopeg- 

 mata (h-h f ), and Ancylobrachia, (n-n'\ 



From the irregularities of the curves made by these sub- 

 ordinal groupings of genera the indications are that the 

 Thecacea (c-c'} is compounded of three distinct groups, hav- 

 ing separate courses of evolution culminating in the Ordivi- 

 cian, in the Carboniferous, and in the Jurassic, respectively; 

 the Trullacea (b-b f ), the Helicopegmata (h-h') y and the An- 

 cylobrachia (-#') are apparently natural groups; at least 

 the present evidence expressed in the structural classification 

 corresponds fairly well with the classification based upon the 

 rate of differentiation of genera within each group. 



The Trullacea are the earliest forms of articulate Brachiop- 

 oda known and their development was earliest of the fami- 

 lies too early for the exhibition of good evidence of actual 

 differentiation ; but the Helicopegmata (/*-//) show the begin- 

 ning of differentiation as late as the beginning of the Silurian 

 era, and as their history has also an ending at about the 

 middle of Mesozoic time, a study of their history should 

 throw some light upon the problems before us. The Ancy- 

 lobrachia (n-n r ) beginning its differentiation about the same 

 time, and continuing to increase, reaching its culmination in 

 the Mesozoic, presents descendants of the old genera and 

 also some new genera even in Recent time. This group of 

 genera may be studied in detail on account of the fuller 



* A Classification of the Brachiopoda, by Charles Schuchert, The Ant. Geolo- 

 gist, vol. xi. p. 141, March 1893. 



