164 Mr. T. R. Jones on Beyrichise. 



modified by a transverse depression on its thickest part, usually 

 forming an oblique indentation on its inner (anterior) side, and 

 producing a bifurcation of the ridge. In the figures in pi. 18 of 

 the 'Mem. Geol. Survey' (above referred to) the artist has in- 

 advertently made the small inner branch of the posterior ridge 

 uniformly continuous with the produced upper part of the ridge ; 

 a condition, however, almost arrived at sometimes by the inden- 

 tation on the broad part of the ridge (compare fig. 3). 



I have not seen the carapace-valve itself of this species ; but, 

 judging from the aspect of the casts and imjjressions it was pro- 

 bably smooth. All the figures arc from casts. Figs. 1-4 are 

 selected from a number of drawings illustrative of the variations 

 of outline, and of the disposition of the ridges. Figs. 1 and 2 

 may be regarded as the typical form. 



In a young specimen from Harnage (fig. 5) the anterior fur- 

 row (dividing the fore and middle ridges or lobes) is seen to be 

 shorter than that between the middle and hinder lobe ; and the 

 indentation on the upper part of the last ridge is vertical and 

 distinct. This specimen is very interesting, as it shows that 

 in the young state this species (so strongly ridged in the adult 

 state) is scarcely removed, except by its well-defined marginal 

 rim, from the merely " crumpled " condition of the Corrugatce. 

 With this young individual several adult specimens occur, which 

 retain the vertical bifurcation of the posterior lobe, as is also seen 

 in Prof. McCoy's figure, Cambridge Pal. Foss. pi. 1 E. fig. 3 

 (which is better matched by our fig. 5 than by the others) ; in 

 other respects they resemble our fig. 1. 



Figs. 1 and 2 are from artificial casts of impressions in dark 

 siliceous micaceous Llandeilo flagstone, from Llan Mill (two 

 miles east of Narberth, Haverfordwest district). Figs. 3 and 4 

 are from internal casts in a dark calcareous Llandeilo flagstone 

 of the same locality. The specimens are very abundant in the 

 shelly bands of the flagstone and scattered over its divisional 

 planes, together with the remains of Trilobites, Leptsena, Encri- 

 nites, &c. (In the Museum of Practical Geology.) 



Fig. 5 is from a soft greenish-yellow argillaceous and mica- 

 ceous bed of the lower J3ala rocks at Harnage, near Shrewsbury ; 

 and occurs with a ^ev^ older individuals, several specimens of 

 B. hicornis, and a vast number of minute specimens, described 

 furth(;r on under the heads of B. strangulatn and B. simplex ; — 

 together with palliobranchiate and other Bivalves*, — nearly all 

 in the state of casts, on the surface of a divisional plane of the 

 rock. (In the Museum of Practical Geology.) 



* Harnage, from wliencc many of these specimens are described, is a 

 rich locality in the Lower Llandeilo (or Bala) flags; Trilobites, including 

 Alenus, are found there. — J . W. S. 



