No. 470] 



OLD AGE IN BRACHIOPODA 



103 



Rhynchotrema capax, No. 142, had in maturity a width of 18 

 im., a length of 17 mm., giving a sliell unlex of 1.00. In old age 

 le width was 20 mm., the length 23 mm., and sliell index 0.87 



In Rafincsquina alt 

 2 mm. in width, 30 i 



No. 1 28, the mature shell measured 

 length, and the shell index was 1.40; 

 I. wide bv 39 mm. loiiff with a shell 



index of 1.30. 

 8. Modifir 



of Prdiclr Opc 



As tlie 



the pedicle normally increases m (hameter it ir contiimcs attached. 

 The resulting growth of the ])edicle mav resorl) the surrounding 

 shell (the deltidium or deltidial plates and umbo) and thus enlarge 

 its opening. This is especially"eonspicuous in the Terebratuloids. 

 In some shells resorption is'made doubly necessary if the pedicle 

 w^ould continue to exist, for the rotation of the umbos toward 

 each other would otherwise soon cut it off. This condition is seen 

 well in some sj)ecimens"ofj/i%nc/io/rma capax. 



Kxamj^les: a specimen of Laquens californicm, No. Tlo, is a 

 senile individual as indicated by its lamellose growth lines, abrupt 

 deflection, and groove at the cardiiuil angles. The uml>o shows 

 considerable resorption as do also 

 the delti<lial plates i Figs. 1 and la). 



A senile si,ccinu>n of Ilrbrrfrlla 



rldru 



The 



has 



large in maturity with the delti- 

 dium resorbed, has been much 

 enlarged in old age; in addition 

 the umbo of the pedicle valve has „„„ i'. ,-uUu-s<- 

 been resorbed, destroying mtich ''""i""'"- 

 more than the nepionic shell (Fig. 

 17). This destruction of the umbo may be 

 breaking as shown by an irregularity at the 

 there is no doubt that most of the opening is 



partially 



side, but 

 » resorption 



