DEVELOPMENT OF THE BRACHIOPODA 



279 



122, A), Glottidia (figure 124, c\ and other higher forms also 

 have corresponding scliizolophian stages, but are without the 

 median septum. Lacazella mediterranea presents a similar 

 larval structure, and in L. Barretti it is retained to maturity. 

 The fossil genera Davidsonella and TJiecidella of the The- 

 cidiidae, and Zellania of the Terebratellidse, never developed 

 beyond the schizolophus-stage, and they must therefore be 

 considered as quite primitive genera in their respective 

 families. 



121 



Taxolophus. 



Trocholophus. 



Schizolophus. 



Ptycholophus. 



Figure 121. — Stages of growth of the lophophore in Thecidea, Cistella, and 

 Megathyris. a, b, c, d, stages in the growth of the lophophore in Thecidea 

 (Lacazella) mediterranea. Enlarged, {a-c, after Kovalevski; d, after Lacaze- 

 Duthiers.) e, f, early stages of lophophore of Cistella neapolitana. Enlarged. 

 (After Kovalevski.) g, adult lophophore of Cistella (C. cistellula). Enlarged. 

 (After Davidson.) h, labial appendages of Megathyris decollata. Enlarged. 

 (After Davidson.) 



From this point the further development and complication 

 of arm structure proceeds in three distinct diverging lines, 

 producing the three characteristic types of brachia of all the 

 higher brachiopods, as exemplified in Thecidea^ Terebratulina, 

 and Rhynchonella. 



