[WILLIAMS] P'OSSIL FAUNAS OF THE ST. HELEN'S BRECCIAS 228 



The plications, in number and strength, are ver}^ similar to those of 

 S p. audaculus and 8. medialis. T,he sinu's of the pedicle valve is broad 

 and its margin slopes gradually into the sides as in 8. granulosus, not as 

 in 8. audaculus. The bottom of the sinus (see PI. I, figs. 10 and 11) is 

 angular, as in some specimens of 8. granulosus and in 8. concinnus. In 

 the early part of the fold of the brachial valve (see PL I, fig. 3, and PL 

 II, figs. 2, 4, 7) there is a sliglit median groove, as in the front third 

 of the shell of »S'. granulosus (see PL I, iig. 8). In young specimens the 

 same groove is visible. The surface of none of the specimens is well 

 enough preserved to determine whether there are pustules or not. In all 

 specimens, well enough preserved to give evidence, the cardinal extrem- 

 ities are extended into short, acute, mucronate points (see PL I, figs. 8 

 and 4). Both valves are highly arched and gibbous, the front of the 

 median part of the pedicle valve is much produced, making the distance 

 from beak to front along the centre line nearly twice as long for the 

 pedicle as for the brachial valve. 



The pedicle valve is more highly arched than in 8p. granulosus, and 

 more like large specimens of 8p. mesistrialis and in this respect ap- 

 proaches the form of 8. acuminatus. The plication's become faint, and 

 in large specifmens are nearly indistinguishable, for the front third part 

 of the surface of the shell (PL I, fig. o). Large specimens in which the 

 plications near the beak are sharp and distinct, are almost smooth in the 

 sinus and central part of the front of the pedicle valve (PL I, fig- 8). 

 The surface is marked by several rather strong lines of growth after 

 mid-growth of large shells. 



The small specimens (PL II, figs. 1-8) are finely plicated and re- 

 semble young specimems of 8. audaculus, but are more arched and with 

 curving area and overarching beaks. 



The species is represented by a series of specimens from small shells 

 less than 10 mm. wide to one fragment of over 50 mm. broad. (See 

 Plates I, fig. 10, and II, fig. 1.) 



The association, mode of occurrence and correspondence of char- 

 acters of the 'specimens of various sizes lead to the opinion that the 

 several specimens here associated form a series of different stages of 

 growth of a single species. 



The combination of characters indicates that 8. montrealcnsis is the 

 forerunner of several distinct species of the later middle devonian beds 

 of the interior of North America, which at this earlier stage were undif- 

 ferentiated. 



One quite small pedicle valve (7 x 12 mm.), showing 13 plications 

 each side the sinus with rather sharp and but slightly arcliing high area, 

 is believed to be a young of this species 8. montrealensis as specimens of 



