166 Mr. T. R. Jones on Beyrichise. 



Salter, Mem. Geol. Survey, 1848, vol. ii. part 1. p. 352. pi. 8. 

 figs. 14, 15, and B. gibbd, ibid. p. 352. pi. 8. figs. 17, 18. B. 

 tuberculata, Siluria, 1854, p. 234 (woodcut 45, 4), pi. 34. fig. 21. 



Surface of valve convex, divided into three lobes; the hind 

 and front lobes both large, p3^riform, but somewhat variable in 

 their i-elative proportions ; the ventral extremity of the anterior 

 lobe extending below that of the posterior lobe ; the middle lobe 

 small, oval, and frequently united with the posterior lobe by a 

 narrow depressed curved neck ; marginal rim distinct. 



The surface sometimes smooth (fig. 7), and sometimes granu- 

 lated (fig. 9). 



]\Iessrs. Salter and M'Coy have given several figures of this 

 species, most of which accord generally with the above descrip- 

 tion, except that for the most part the anterior and posterior 

 lobes are made to appear continuous along the ventral part of 

 the valve. M^Coy^s figure 2, pi. 1 E. Brit. Pal. Foss. is an in- 

 teresting exception to this condition; and it differs from our 

 specimens in having the anterior lobe much reduced in width. 

 Salter's figures 17 and 18. pi. 8. Mem. Geol. Surv. are excep- 

 tional also in the angular production of the ventral border; these 

 are from the iMiddle or Upper Silurian series at the Slate Mill, 

 S.W. of Haverfordwest. 



M^Coy also mentions [loc. cit. p. 136) a well-marked variety, 

 having a long central ridge continued to the ventral * border 

 from the Bala schist at Dermydd Fawr, near Craig Bronbanog, 

 N.\y. of Corwen. 



Fig. 6 is an artificial cast of an impression in calcareous flag- 

 stone (in the JNIuseum of Practical Geology), which contains 

 numerous specimens of this species, chiefly on the divisional 

 planes, with remains of Trilobites, Leptfena, &c., and belongs to 

 the Wenlock shale of Tynewydd, S. of Llandovery. Fig. 9 re- 

 presents the exterior of a well-preserved specimen (in Mr. Mor- 

 ris's collection) from the Wenlock limestone of Lincoln Hill, 

 near Dudley. 



In addition to the above-mentioned localities for this species 

 Sir R. Murchisont gives " Tilestone (Upper Ludlow), Lodge 

 Bank, Downton ;" JMr. Salter mentions W'oolhope (Wenlock 

 limestone) ; and Prof. M'Coy enumerates Underbarrow, Kendal, 

 Westmoreland (Ludlow Ilocks) ; Cowan Head, Kendal (Upper 



* In the comparison of tlie species here described, with the descriptions 

 by Prof. M'Coy, it shoidd be ol)served that I regard as the dorsal border 

 that which M'Coy desciibes as ventral, and vice versa. 



t This sjjccies " is very al)un(hint fr()ra the base of the Wenlock shale to 

 the highest Lndlow stratum, and is a good index of Upjier Sihu-ian rocks, 

 tliough found sometimes iu the upper division of the Caradoc." — Sihiria, 

 p. 2'M\. 



