110 



THE AMERICAN NATURALIST 



[Vol. XL 



becomes much thickened and more or less grooved by vascular 

 markings. Both valves of this species bear lamellose growth 

 lines. There is also a greater lamellositj in the latest built por- 

 tion of the deltidium and chilidium. 



Productus horridus Sowerby. — No. 600 from the Lower Zech- 

 stein of Gera, Thuringia, is not very senile. Old age, however, 

 is indicated by the greater concavity of the brachial valve and 

 by the absence of spines from the last added 12 mm. of the ante- 

 rior portion. They had disappeared before this from the car- 

 dinal angles, showing thus the progressive advance of senility 

 from the cardinal angles to the anterior portion. This order of 

 disappearance is just what we should expect from their order of 

 initiation, appearing as they do at the cardinal angles before they 

 develop on the main portion of the shell. 



Platystrophia lynx (Eichwald). — A gerontic individual of this 

 species from the Hudson River group of Cincinnati, Ohio, No. 

 1911, M. I. T., attained a length of 31 mm. before it began to 

 show evidence of old age in any marked degree; after this point 

 it added a length of 13 mm. to each valve. The senescent char- 

 acters noted here are: lamellosity of growth lines, flattening of 

 the ribs, and formation of a groove at the cardinal angles. Through 

 the change in the angle of curvature and the consequent growing 

 toward each other of the two valves, the entire shell becomes very 

 gibbous. The accentuation of these characters in increasing old 

 age may be seen by comparing Figs. 9 and 10. The cardinal angle 

 measures 78° at the close of the ephebic stage and 94° in the geron- 

 tic. Similar observations are given by Cumings (:03, p. 12). 



A pedicle valve of this species. No. 3, from the Hudson River 

 group of Cincinnati, Ohio, shows a very pronounced thickening 

 on each side of and anterior to the muscular area. This area 

 thus appears to be very much depressed, with high, perpendicular 

 bounding walls. The development and relationship of this species 

 are very thoroughly discussed and illustrated by Cumings (:03). 



Rhjnchotrema capax (Conrad). — A specimen of this species. 

 No. 142, from the Hudson River group of Cincinnati, Ohio, after 

 reaching maturity when it had a length of 15 mm., became lamel- 

 lose at the cardinal angles and the ribs began to flatten out, while 

 the angle of curvature in each valve became relati\ely greater. 



