No. 470] 



OLD AGE IN BRACHIOPODA 



111 



It then lengthened the pedicle valve 8 mm. on the curve of the 

 shell, during ana- and metagerontism. At this point the growth 

 lines become still more lamellose, more of the ribs flatten out, 

 and a sudden increase in the angle of curvature takes place. From 

 this point it added 7 mm, to each valve. There is shown especi- 

 ally on the median sinus and folds of this latest added portion a 

 groove in the center of each rib on account of the changed plane 

 of growth. A shallow groove is formed at the junction of the 

 valves. This is greater at the cardinal angles since it is there 

 first formed and proceeds progressively anteriorly. This groove 

 is due in the anterior portion to the last added two or three growth 

 lines only. The specimen has the pedicle opening entirely filled 

 by the umbo of the brachial valve. Another specimen, however, 

 No. 1156, as large as the preceding and 

 representing a similarly advanced stage of 

 senescence, has a very large pedicle o]:»en- 

 ing. Mature and gerontic forms are shown 

 in Figs. 7 and 8. 



Terehraiula Imrlani Morton. — In this spe- 

 cies old age is shown in the lamellose con- 

 centric growth lines, the change in the angle 

 of curvature, the groove at the cardinal 

 angles, the larger cardinal angle, and the re- 

 sorption of the umbo and dcltidial plates. 

 Exactly similar characters are shown in T. 

 pcrovalis Sowerby. 



Tropkloleptv,'} cannatus ((onrad).— In 

 an old-age specimen of this species, No. 

 1915, M. I. T., the ribs are flattened out on 

 the gerontic portion and the growth lines are lanu^llosc. irrcirnlar, 

 and more or less piletl up. These senile cliaractoi> a[)|)car pro- 

 gressively from the cardinal aui^le to the front of th*- >h<"l! ( Fii:. 

 25). The cardinal angle enlarut- .u,<l thr ^lirll iiul. x ^inu . smaller. 

 The ribs flatten out on the l)racliial valve \)vUn-v tiirv do on the 

 pedicle vahe. In odier ^pedintMi- the Mil.inaiLin.il of the 



concave (brachial) \ahe, so chara( reii.tic of liaiiiu ^^jiuna alhrnata, 

 also occurs, though in a less marked degree. Kaynioud ( : 04, 

 pp. 12G-131) discusses this species fully. 



