262 



STUDIES IN EVOLUTION 



freys, which lack deltidial plates in the adult shell. The 

 young of other telotremate species, as Magellania flavescens 

 or Terebratulina septentrionalis, agree in the same respect. 



114 



119 



115 



-Id 



118 



120 



116 



FIGURE 114. Delthyrium of young Rhynchonella, without deltidial plates. 



FIGURE 115. The same at a later stage, with two triangular deltidial plates. 



FIGURE 116. The same after completed growth; showing joining of 

 deltidial plates, and limitation of pedicle-opening to ventral beak. 



FIGUBE 117. Dorsal view of Magellania Jlavescens ; showing completed 

 deltidial plates, del. 



FIGURE 118. The same; profile, ds, dorsal valve; vs, ventral valve; p, 

 pedicle. 



FIGURE 119. Dorsal view of umbonal portion of adult Terebratulina 

 septentrionalis, with shell removed by acid; showing slight secondary exten- 

 sion of ventral mantle around pedicle (consequently small deltidial plates are 

 secreted in this species). Mantle areas secreting deltidial plates are shaded. 



FIGURE 120. Dorsal view of umbonal portion of Magellania Jlavescens, with 

 the shell removed by acid ; showing the complete envelopment of base of pedicle 

 by secondary expansions from ventral mantle, and consequent production of 

 deltidial plates filling delthyrium except at pedicle-opening. See figure 117. 



An examination of .the animal at this stage shows that the 

 mantle lobes line only the interior of the valves proper. 

 The exposed edges of the mantle are around the peripheries 

 of the valves and also that portion of the ventral mantle 



