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THE AMERICAN NATURALIST [Vol. XL 



Laqueus calif ornicus Koch. — A specimen of this recent species, 

 from Catalina Island, California, No. 715, shows old age in the 

 lamellose growth lines, the change in the angle of curvature, and 

 the formation of a groove at the cardinal angles, and in the larger 

 cardinal angles (Figs. 1 and 2). These characters also appear in 

 L. rvbellus Sowerby, etc. 



Atrypa spinosa Hall. — In a specimen from the Hamilton of 

 Eighteen Mile Creek, New York, No. 499, old age is first indicated 

 after a growth of 27 mm. by the sudden crowding together of the 

 growth lines and by the change in the angle of curvature. After 

 this the anterior growth through the change in curvature adds 

 about 5 mm. to the thickness of the shell and 4 mm. to its length 

 measured along the antero-posterior axis. This gives the shell 

 a very gibbous appearance. Senility is first expressed at the 

 cardinal angles as seen in the development there first of the greater 

 lamellosity of the growth lines, the change in the angle of curva- 

 ture, and the formation of a groove which does not extend far 

 anteriorly. The thickness of the comparatively flat pedicle valve 

 is much greater than that of the brachial. For comparison of 

 the senile characters of this specimen with an adult form, see 

 Figs. 3 and 4. A pedicle valve. No. 641, shows the separation 

 of the muscular area from the rest of the valve by a thick, promi- 

 nent ridge which is especially developed at its sides, i. e., over 

 the genital organs (Fig. 20). The thickened lateral edges of this 

 form a prominent inflected edge. 



Spirifer mucronatm var. thedfordense Shimer and Grabau. — 

 A specimen of this species. No. 405, from the Hamilton group 

 of Arkona, Ontario (Fig. 16), shows old age in an increase of the 

 lamellosity of the concentric growth lines, the fading-out of the 

 ribs, the change in the angle of curvature, and the development 

 of a groove cardinally at the junction of the two valves. These 

 characters hold true in all senile specimens of this genus examined. 

 In S. acuminatus the greatest thickening of the valves is on each 

 side of the muscular area. This character appears to hold true 

 in all species. In some species (e. g., enrijteines, acuminatus, 

 oweni, etc.) a conspicuous thickening (callosity) occurs also pos- 

 terior to the muscular area in the pedicle valve, thus separating 

 it widely from the apex of the valve (Fig. 19). 



