262 



STUDIES IN EVOLUTION 



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

 young of other telotremate species, as Magellania flavescens 

 or Terehratulina septentrionalis, agree in the same respect. 



114 



117 



119 



115 



118 



120 



FiGDEE 114. — Delthyrium of yonng RhynchoneUa, without deltidial plates. 

 , FianEE 115. — Tlie same at a later stage, with two triangular deltidial plates. 



FiGnKE 116. — The same after completed growth; showing joining of 

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



FiGUKE 117. — Dorsal view of Magellania fiavescens ; showing completed 

 deltidial plates, del. 



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

 pedicle. 



Figure 119. — Dorsal yiew of umbonal portion of adult Terehratulina 

 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 Magellaniajlavescens, 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 fiUiug 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 



