262 



STUDIES IN EVOLUTION 



freys, -wliich lack deltidial plates in the adult shell. The 

 young of other telotremate species, as Mayellania fiavescem 

 or Terehratulina septentrionalis^ agree in the same respect. 



114 



117 



119 



115 



116 



Figure 114. — Delthyrium of young RhynchoneUa, without deltidial plates. 



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



Figure 116. — The same after coini)leted growth; showing joining of 

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



Figure 117. — Dorsal view of MiKjellania fiavescens; sliowing 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 i)lates are 

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



Figure 120. — Dorsal view of umbonal portion of Ma<iellaniajlavescens, with 

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

 by secondary expansions from ventral mantle, and consetpient production of 

 deltidial plates filling delthyrium e.xcept 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 ])eripheries 

 of the valves and also that portion of the ventral mantle 



