No. 470] 



OLD AGE IN BRACHIOPODA 



117 



odontas, as noted above (section 10), these papillose protuberances 

 become almost spine-like. 



The principal thickening in at least many brachiopods occurs 

 over the main trunks of the vascular sinuses (compare Figs. 27, 

 28, 29; see also Fig. 22). It is a significant fact that in these are 

 located the genitalia (Hancock, '59, p. 817). If a greater con- 

 traction took place there it is just in line with what we know occurs 

 in higher animals (Quain, : 03, p. 1478). It is usually held that 

 "no gerontic limit is known to the reproductive time in the lower 

 animals" (Hyatt, '97, p. 220). As there is doubtless in most 

 shells an increasing amount of space unoccupied by the soft por- 

 tion of the animal as it increases in age, it is not necessary to 

 postulate a great shrinkage of the soft tissues to account for the 

 thickening of the shell. Yet the fact remains that in many species 

 the principal thickening is over the main trunks of the vascular 

 sinuses, just where the genitalia occur in modern species and 

 where very probably they were located in fossil ones. 



The greater reduction of the lateral growth of the mantle over 

 that of anterior growth in brachiopods is seen in the fact that in 

 old age the shell is proportionally longer than in maturity. The 

 result of these old-age processes appears first at the cardinal angles 

 where the loss of lateral growth to compensate for the shrinkage, 

 causes the flattening-out of the mande folds (see also Williams, 

 '95, p. 309). The reduction of the radial ribs proceeds progres- 

 sively from the cardinal angles to the anterior border of the shell 

 and hence it is on the sinus and fold that w^e find the ribs persist- 

 ing strongest. 



In those cases where the ribs flatten out entirely their continu- 

 ance is indicated by zigzag lines of growth on the smooth surface 

 of the gerontic portion of the shell. These sliow tluit tlic mantle, 

 after flattening out on one plane, still retained the scallopcf] honh^r 

 on another. This scalloped edge (as seen for example in /.'//////- 

 chotrema capax) resulted from the faster growing of |)arts of the 

 mantle over others. As the mande curved, the parts which formed 

 the summits of the ribs fell behind those which formed the depres- 

 sions. In other words the portions in the depressions grew faster. 

 This difference in the rate of growth may be seen by following 

 two ribs and their included sinus from the umbo to the front of 



