﻿Vol. 6 1.] 



SEQUENCE m THE BKISTOL AREA. 



293 



The median ridge in the interior of the concave valve is grooved, 

 as in fig. 8 a of the same plate. 



Prom the coarseness of the ribbing, this may be called the 

 costate form, since the specimens bear a strong resemblance to 

 Productus costatus. There is a variety in which the ribbing is very 

 much finer and the semireticulation much clearer; this I term the 

 semireticulate variety. 



Specimens occur in D 2 and e, but the maximum is in e. 



Note on the Position of the Adductor-Impressions 



in the Convex Yalve. 

 The strain on the muscle at its point of attachment to the convex 

 valve must be in the direction of the stress caused by contraction, 

 and hence must act along the muscle. This strain will naturally 



cause displacement 

 Fig. 3. — Muscular scars in the convex valve. f the area of at- 

 tachment, until the 

 stress is at right 



Productus 0. S,. 



Productus Cora [Da v.]. 

 Mutation from S. 



T Productus 



\ aff. semireticulatus. 



[ Si. 



a — Adductors. 



Diductors. 



angles to that area. 

 Hence, if the valve 

 to which the ad- 

 ductor is attached 

 becomes more con- 

 vex, there will be 

 a shift of the area 

 of attachment to- 

 wards the beak : 

 the reverse shift 

 taking place in a 

 flattened or sulcate 

 valve. For this 

 reason, the relative 

 positions of the 

 adductors and di- 

 ductors is similar 

 in the convex forms 

 Productus gigan - 

 teus, Pr. Cora, Pr. 0, 

 and Chonetes co- 

 moides, but differs 

 from that observ- 

 able in the sulcate 

 forms of Productus 

 semireticulatus. The 



accompanying diagrams (fig. 3) illustrate this point (see also under 

 Chonetes aff. comoides, p. 295). 



Chonetes. 

 Chonetes cf. hardeensis, Phil. (PI. XXVI, figs. 1 & 2.) 



I have retained the name hardrensis, because it does no more 

 Q. J. G. S. No. 242. * 



