106 



THE AMERICAN NATURALIST [Vol. XL 



Productus horridus, No. 600, 43 mm. wide and with a length of 

 82 mm. following the curve of the pedicle valve, has no spines 

 on the last added 12 mm. of the anterior portion, while the spines 

 had disappeared earlier from the surface at the cardinal angles 

 (see p. 110). 



Another specimen of the same species, No. 607, has no spines 

 on the last added 18 mm. This disappearance of spines in old 

 age is also well seen in Amhocalia sfinosa and in Atrypo spinosa. 

 In Atnjpa nodosfriafa the disappearance of nodes from the senile 

 portion of the individual was noted. 



10. Thickening of Valves — This may result in the formation 

 of an elevated ridge al)out the muscular area and in the building 

 of a ridge just inside the margin of the concave valve in concavo- 

 convex forms. Both valves, and especially the pedicle valve, 

 thicken by interior additions. The area of maximum increase 

 usuallv extends from each side of the muscular impression to the 

 cardinal angles. Sometimes, as for 

 example in Afhyris spiriferoides , the 

 greatest thickening is at the lateral edges 

 of the valves. The pedicle valve be- 

 comes especially thickened over the 

 gential organs as seen in Atrypa, Spirifer, 



Examples: a pedicle valve of Atrypa 

 reticularis, No. 641, has a broad, promi- 

 nent ridge bounding the muscular area 

 laterally and sloping outward to a de- 

 pression between it and the much 

 thickened and inflected edge of the shell; 

 it disappears entirely anteriorly (Fig. 20). 

 A pedicle valve of Spirifer acuminatus, 

 No. 646, shows a greatly depressed muscular area due to the great 

 thickening of the posterior portion of the valve on each side of 

 it, which slopes gradually to the lateral margins of the valve (Fig. 

 19). 



In Platystrophia lynx, No. 3, the pedicle valve is thickened 

 very much at the sides of and anterior to the muscular area (see 

 also Cumings, :03, p. 28). 



