146 C. E. Beecher — Development of the Brachiopoda. 



ventral valve occupied by the pedicle. This condition repre- 

 sents the extent of the development of these parts in Meris- 

 tina rectirostra Hall or Gwynia capsula Jeffreys, which lack 

 deltidial plates in the adult shell. The young of other telo- 

 tremate species, as Magellania flavescens or Terebratulina sep- 

 tentrionalis, agree in the same respect. 



33. 



35. 



Figure 30. — Delthyrium of young Rhynchonella, without deltidial plates. 



Figure 31. — The same at a later stage, with two triangular deltidial plates. 



Figure 32. — The same after completed growth; showing joining of deltidial 

 plates, and limitation of pedicle opening to ventral beak. 



Figure 33. — Dorsal view of Magellania flavescens ; showing completed deltidial 

 plates, del. 



Figure 34. — The same ; profile, ds, dorsal valve ; vs, ventral valve ; p, pedicle. 



Figure 35. — Dorsal view of umbonal portion of adult Terebratulina septentrionalis. 

 with shell removed by acid; showing slight secondary extension of ventral 

 mantle around pedicle (consequently small deltidial plates are secreted in this 

 species). Mantle areas secreting deltidial plates are shaded. 



Figure 36. — Dorsal view of umbonal portion of Magellania flavescens, 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 33. 



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 border limit- 

 ing the deltidial opening and passing along the sides of the 

 pedicle at its base. The ventral mantle gradually extends 

 from each side as two prolongations partially covering the 

 opening and enveloping the proximal portion of the pedicle. 



