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 



122 



Taxolophus. 



Trocholophus. 



Schizolophus. 



Zugolophus. 



Plectolophus. 



FiGfRE 122. — Stages of growth of the lophophore in the Terebratellidae and 

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

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

 e, Magellania kerfjnelenensis. Natural size. (After Davidson.'^) f,g, h, >,j, develop- 

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

 septentrional is. Enlarged. (After Morse.^i) j, adult Terebratulina cancellata. 

 (After Davidson.'') 



called the jjti/cholophus. Megathjiris and Lacazella medifer- 

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

 has six; T. vei'micularis and Uudesella mai/ale, eight; 

 B. dit/itata, ten ; Pterophloios and Oldhamina, about twenty. 

 Lobation in some (^Thecidea) is produced by the forking or 



