No. 470] 



OLD AGE IN BRACHIOPODA 



101 



4. Flattening-out of Plications. — In old age the plications (ribs) 

 tend to flatten out and disappear. Their presence is usually 

 indicated on the gerontic portion of the shell by zigzag lines of 

 growth, though the surface of the shell is at this area smooth. 

 ^Fhis is seen in UhiinMrema capax, Plafi/.sfmphia l/pi.T, Twp!- 

 dolepius cnrlnahis, SpirifcM's, etc., and holds true in all spccinuMis 



I'XampK-.: in lllnjrrhntr, mn mpax, Sn. Ill', fn.ni Cincinnati. 

 Ohio, the i)cdicle valve has dnrino' maturity l.") ril.s; durin- nicta- 

 gerontisni it has 11, and at the last uTowth bcfon^ the death of 

 the animal there are n(nu\ allliouu-li zigzau' <:ro\vth lines repnvseiit 

 them. The brachial valve has during maturity 14 ribs; during 

 metagerontism, U), with none at tlie death of the animal. 



In Spirifcr oweni. No. 57, from the Ilamiltcm of (Mark Co., 

 Indiana, the ribs l)ecome broader and lower until in extreme old 

 age they, as well as the zigzag growth lines at the edge of the shell, 

 have almost entirely disai)i>eared even at tli(> anterior portion of 

 the shell (see ]>. 117 for further discussion). 



The ribs disappear .-arlier and. more compl(>telv from the brach- 

 ial than from the pedicle xah,>. Thi. ua^ noticed especially in 

 Trrrhralrlla pllrala Sa\ . rrnpl,lnh phi.- rarnnlns ("onra-l , .1/../.- 



(Conrad). This character is often only 

 marked; its presence is first noted at tli 

 nal angles, llaytnond ( :()4. p. ll'M. als 



w"////.s- am/ Fohl. 

 id to (latt(>n out 



(Fig. I 1 , the median .im.. 

 In B;iulut,s rariru.s .Conrii 

 dencyto obliterate tlH> man 

 of shells, Xo. 4. from th( 

 Albany Co., New ^'ork ( 

 also needier (:()!, p. 40;] 



