no. 1646. REVISION OF BETRIOHIIDJE—ULRICH AND BASSLER. 287 



to regard Beyrichia as essentially a north European genus. The 

 direct opposite is true of the group of Beyrichiidae typified by />. 

 wilckensiana Jones, for which Jones and Tloll subsequently erected 

 the genus Klcedenia. That is to say, the latter genus is much more 

 Strongly developed in America than in Europe. 



About ninety of the species originally described as Beyrichia re- 

 main to be distributed among their proper genera. A large pari of 

 this number will be accounted for in the following discussions of the 

 other genera of the family. But a considerable number will remain 

 even then that for one reason or another cannot yet be definitely 

 placed. In most cases it is lack of knowledge that suggests delay 

 in deciding their systematic positions. A few, however, require 

 further study and comparison, being too peculiar to fall readily into 

 place. 



The species here definitely referred to Beyrichia fall into seven. 

 in part genetic, in part perhaps artificial groups. The first three of 

 of these, the B. Moedeni, the B. tuberculata, and the B. buchiana 

 group-, are made up entirely of unequivocal species of the genus. 

 The other four groups, however, are more or less synthetic, and, 

 though including species that cannot be clearly distinguished from 

 Beyrichia, they are yet closely connected with species that must be 

 referred to other genera. In other words, they represent different 

 lines of development that seem to have originated in diverse 

 Ordovician types but ended through what might be called convergent 

 evolution in much more uniform stages. It appears further that in 

 the decline of the genus a partial reversion to ancestral stages took 

 place. Suggestive observations bearing on these points will be found 

 in the following notes. 



GROUP OF I'.. KLCEDENL 



This group embraces B. klosdeni, 11. maccoyiana, II. bolliana, B. 

 icochii, 11. tuberculato-kochiana, B. moodeyi, new species, />'. lind- 

 \fromi, and most of the forms described by various authors as van- 

 ities of the first-named species. Its valves are usually short, semi- 

 lliptical in outline, and sharply lobed. The lobes are of medium 

 hickness, the anterior one tending to enlargement and dissection, 

 vhile the middle lobe is nearly as large as the posterior and com- 

 nonly exhibits a tendency to union with the incurved ventral extrem- 

 ty of the anterior lobe. The posterior lobe is more or less acuminate 

 ind incurved below, but does not extend forward beyond the base of 

 he median lobe. The type usually is shorter and always has some- 

 vhat thicker and more bulbous lobes than the buchiana section, and 

 t is commonly shorter, with less dissected anterior and posterior 

 obes, and a longer median lobe than the tuberculata group. It con- 

 titutes, therefore, an admirable central type for the genus, and its 



