[WILLIAMS] FOSSIL FAUNAS OF THE ST. HELEN'S BRECCIAS 237 



mens of this valve in the same condition of preservation differ by having 

 the body of the shell ^nore elevated over the centre, the shell thicker over 

 the cardinal border, and the front margin narrowing less rapidly ('see 

 plate III, fig. 1) toward the cardinal angles, giving the body a more 

 cylindrical form than the typical species. Also the muscular scar of the 

 pedicle valve under the beak is much broader and more strongly deve- 

 loped, and the median sinus is more widely expanded toward the front 

 (Plate III, fig. 13 and Plate IV, figs. 4 and 14). One specimen (see 

 plate III, fig. 10) is 38 mm. from centre to the exti-emity of the mucro- 

 nate point on the left side, or its total length was 76 mm., its length from 

 tip of beak to front is 18.4 mm. The muscular scar is 10.6 mm. wide at 

 base and 8.8 long, and the sinus is 9 mm. wide at front. Another speci- 

 men (plate IV, fig. 14) shows a muscular scar 12-8 wide at its base. The 

 muscular scar of the pedicle valve is deeply impressed and in molds of 

 the interior shows the prominent tongue-like "process" bounded by the 

 dental lamellae. This character is produced by the thickening of the 

 shell along the cardinal margin outside the dental lamellœ. This thick- 

 ening of the shell along the cardinal border up to the teeth plate's, with 

 deep sinking of muscular scar in the shell is characteristic and associates 

 the species with the other specie's of the lower Devonian such as S. cum- 

 herlandiœ and 8. murchisoni and S. gaspensis of the same fauna and 

 differentiates them from the closely related forms of the Middle Devo- 

 nian. It is also less prominent in *S'. macra. 



The shell is thicker and more convex than is common with the long 

 winged forms of Sp. pennatus, thus resembling S. [macra, but 8. Helenœ 

 differs from the latter by its slightly elevated hinge-area (see PI. Ill, 



fig. 13.) 



The hinge-area is 'somewhat higher in the centre than at the wings, 

 showing in this respect intermediate state between 8. pennatus and 8. 

 macra, but the height of the largest specinuen (fig. 14) showing area is 

 only 3.3 mm. immediately under the beak. Also, the thickening of the 

 shell along thet hinge margin produces a more prominent and full beak 

 of the pedicle valve than is seen in the broad-winged form of 8. pennatus 

 of the Hamilton. 



The number of plications and their size are very similar to those of 

 the Hamilton form of 8. pennatus. 



The mesial sinus of the pedicle valve is bounded by strong plica- 

 tions which exceed in strength and elevation the neighbouring ones of the 

 sida (see PI. IV, figs. 2 and 7). 



Â& in 8. mucronatus there is indication of a faint plication in the 

 bottom of the fold the pedicle valve, whicli is made more evident in speci- 

 mens (see plate IV, fig. 15) with shell surface preserved by the back- 



