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. 



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

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

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

 e,Magellaniakerguelenensis. Natural size. (After Davidson.') _/]j, ^, i,y, develop- 

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

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

 (After Davidson.') 



called the ptycholophus. Megaihyns and Lacazella mediter- 

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

 has six; T. vermicularis and Eudesella mat/ale, eight; 

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

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



