482 Forty-fifth Report on the State Museum. 



deltidial plates are attained at the maturity of different 

 species. (Plate 23, figs. 11 and 12.) Thus, in Meristina 



a£ 



Figs. 60, 61, 62,—Whitfieldella nitida ; showing the progres- 

 sive closing of the deltidium hy the growth of the deltidial 

 plates. The figures at the right represent immature con- 

 ditions, that at the left the adult state. 



Fig. 63. — The adult condition 

 of the delthyrium in Meris- 

 tina rectirostra, showing the 

 absence of deltidial plates. 



rectirostris they are but feebly, if at all developed, evinc- 

 ing either the embryonic or degenerate char- 

 acter of the adult in this respect ; while in Eume- 

 tria Verneuiliana they are closely united by 

 coalescence from the apical foramen to the hinge 



fig. 64.— Rhyncho. and their convexity wholly obliterated. 

 neiia Grasiana, With present evidence, these deltidial plates 



showing the ex- L 7 ■ r 



travagant growth appear to be a secondary modification of the delthy- 

 °, f . the . 1 delt i di ,! 1 rium, induced after the disappearance of the origi- 



platesm the adult. ' i ' ° 



(Dkslonqchamps.) nal covering of this passage from the causes 

 already suggested. 



In many of the articulate genera the delthyrium, if left open 

 either from resorption of its normal coverings or from a failure 

 to develop them, becomes filled by a progressive secretion of 



testaceous matter in the later stages 

 of growth. This is especially marked 

 among a certain group of the Spiri- 

 5 : 7ML1^S^S^. fers where this callous growth forms 



a more or less complete transverse 



Fig. 65 — A cardinal view of Spirifer , , . , „ , . 



asper; showing the callosity in the plate ; and the extreme result ot this 



delthyrium. development is exhibited by the 



genus Syringpthybib, in which this condition of hypertrophy 



in the shell glands, produces not only a complete transverse 



plate covering or partial filling of tin 1 cavity extending from 



34 



