108 



THE AMERICAN NATURALIST 



[Vol. XL 



Hall, and »S. concava Hall. Sometimes the papillae are so well 

 developed that they are spine-like. This was most conspicuous 

 in Stropheodonta magniventra, No. 1165, and in >S. concava, No. 

 1099, M. I. T. 



In the majority of brachiopods the muscular area of the pedicle 

 valve becomes in gerontic individuals depressed below the general 

 interior level of the shell through the thickening of the shell about 

 it, and thus frequently leaves this area translucent in its thinness 

 while the remainder of the shell is very thick. The area is often 

 strongly marked off from the rest of the valve by an elevated ridge 

 at times high and well defined, surrounding it. This ridge is 



conspicuous in Rafinesquina alternata, Leptwna rhomhoidalis , 

 Hebertella occidentalis (Fig. 24), Eatonia peculiaris, and Hip- 

 parionyx proximm, in all of which the muscular area is strongly 

 marked off from the rest of the shell. In Spirifer acuminatus 

 (Fig. 19) the ridge surrounding the muscular area is merged with 

 the general thickening of the shell. We have not been able to 

 examine any senile forms in wliich th(> dental lanielhe of the pedicle 

 valve are strongly developed and fctnn :i spondvlium, as for ex- 

 ample in Pentarnerus, Gypidiihi, v\c. In rluvsc the muscular 

 area is thus raised instead of retaining its youthful position. This 

 thickening of the valves may in a few inchviduals result in the 

 lessening of the total capacity of the body chamber. Usually, 



