No. 470] 



OLD AGE IN BRACHIOPODA 



109 



however, the internal thickening is more than offset by the growth 

 of the margins of the valves toward each other; for even a slight 

 marginal growth means a large increase in the cubic capacity 

 of the shell. 



Descriptions of a few Species 



The following species were chosen for description of senescence 

 because there were gerontic specimens of them in the collections 

 studied and also because they are common. Similar old age 

 characters were, however, noted upon all forms which showed 

 any approach to gerontism. When one specimen is described, 

 this is merely taken as a type but the characters hold true for all 

 the specimens of that species examined. 



Rafinesquina alternata (Emmons). — A large specimen. No. 

 128, from the Hudson River group of Cincinnati, Ohio, was 30 

 mm. long when it first showed signs of old age in the appearance 

 of lamellosity and in the changed angle of curvature; this change 

 is much more noticeable on the pedicle than on the brachial valve. 

 The shell also increases in thickness from 3.5 mm. in a normal 

 mature specimen to 11 mm. in this gerontic individual. In old- 

 age specimens of this species the pedicle opening is usually en- 

 tirely closed and if it exists, is much too small to admit the passage 

 of a pedicle large enough to support a shell of such a size (on this 

 point see also Hall and Clarke, *92, p. 141). This condition is 

 not, however, due wholly to senility but existed during maturity. 



The interior of the brachial valve has, extending from the car- 

 dinal angles around the margin of the valve anteriorly, a tumid 

 ridge with a very irregular surface. This descends abruptly to 

 the edge of the valve. For comparison of mature and gerontic 

 forms see Figs. 21 and 22. 



Strophome7ia rugosa Blainville (Sfrrptorhi/ncJtu.s planumhonm 

 Hall). — A pedicle (concave) valve, No. 582, of this species bears 

 just inside its edge a tumid ridge with a considerable vascular 

 surface, which is not developed to such an extent as in the concave 

 (brachial) valve of Rafinesquina alternata. This fact is interesting 

 as the ridge is developed in opposite valves in the two species. 

 The muscular area remains translucent while the rest of the valve 



