172 Mr. T. R. Jones on Beyrichlre. 



variation in the shape of the valve ; and^ what appears to me to 

 be of more importance, the marginal rim is in one variety highly 

 developed (as in the type), and in others it is absent ; nor do I 

 find any reason to suppose that these latter individuals have lost 

 their rims accidentally. 



B. strangulata, var. a. PL VI. fig. 19. 



Valve much less quadrate than the typical form ; dorsal furrow 

 faint, extending across two-thirds of the width of the valve (an- 

 other but very faint impression occurs on the anterior part of the 

 valve, but amounts to little more than an undulation in the ge- 

 neral convexity of the surface) ; marginal rim strongly developed 

 at the infero-anterior border, and tapering off posteriorly. 



Fig. 19 is from a unique cast in Lower Silurian fossili- 

 ferous schist from Robeston Wathen, Pembrokeshire. (Mus. 

 Pract. Geol.) 



B. strangulata, var. B. PI. VI. figs. 20, 21. 



Carapace-valve (adult) narrow- oblong, with the ends rounded 

 and the anterior extremity contracted ; surface coarsely pitted ; 

 dorsal furrow short and deep, in the middle of the anterior half 

 of the valve ; anterior side of furrow slightly elevated in some of 

 the casts, but not in the impression made by the outside ; no 

 trace of marginal rim. 



Four specimens of this form occur (as casts or impressions) in 

 dark-coloured fossiliferous schist, of Lower Silurian age, from 

 Sholes Hook, Haverfordwest. (Mus. Pract. Geol.) 



The cast and impression of a young individual (fig. 21) ac- 

 companies the foregoing. It is proportionally broader and 

 shorter; nearly semicircular, but obliquely acute anteriorly; the 

 sulcus is well defined, and its anterior edge is raised (in the 

 cast) ; the surface appears to be smooth ; there is no marginal 

 rim. 



B. strangulata, var. 7. PI. VI. fig. 22. 



Cast of carapace-valve very small ; convex ; narrow-oblong, 

 narrower in front than behind; dorsal furrow short, strongly 

 marked, and accompanied by an anterior tubercle ; no marginal 

 rim. 



From Harnage, Shrewsbury ; and accompanies B. complicata 

 (fig. 5) and others : see p. 165. 



With regard to B. strangulata and its varieties above men- 

 tioned, I would observe that M. Barrande's collection of the 



