280 



STUDIES IN EVOLUTION 



Ptycholophus Stage. 



The simplest of the types of brachia just cited is developed 

 out of the schizolophus by the additional lobation, or loop- 

 ing, of the primary lobes, making a structure which may be 



Taxolophus. 



Trocholophus. 



Schizolophus. 



Zugolophus. 



Plectolophus. 



FIGURE 122. Stages of growth of the lophophore in the Terebratellidje and 

 Terebratulidae. a, 6, c, d, e, five stages in the development of the lophophore in 

 the Terebratellidae. a-d, Terebratalia obsoleta. Enlarged. (After Beecher. 2 ) 

 e, Magellania kerguelenensis. Natural size. (After Davidson. 7 ) /, g, h, i,j, develop- 

 ment of lophophore in the Terebratulidae. /-, early stages in Terebratulina 

 septentrionalis. Enlarged. (After Morse. 11 ) /, adult Terebratulina cancellata. 

 (After Davidson.7) 



called the ptycholophus. Megathyris and Lacazella mediter- 

 ranea both have four lobes (figure 2, d, ); Thecidea radiata 

 has six; T. vermicularis and Eudesella may ale, eight; 

 E. digitata, ten; Pterophloios and Oldhamina, about twenty. 

 Lobation in some (Ttiecided) is produced by the forking or 



