166 DE. a. L. ELLES OS THE BALA COUXTEY : [vol. Ixxviii, 



The relative proportions of the two shells are markedly 

 different : — 



In (a) the hinge-line is not the greatest width of the shell and the length 

 is approximately the same as the width, which is greatest at the middle 

 of the shell : the angles are obtuse, the pedicle- valve is definitely but 

 gently convex, the brachial valve being nearly flat or very slightly 

 convex, and showing a distinct but single cardinal process. 



The sti'ucture of the shell is very punctate in between the ribs, and 

 the ribbing characteristically fasciculate. The transverse striations 

 are very clear when the shell is well preserved. 



This seems to be the true Ovtliis liivnanfensis M'Coy. 



In (h) the shell is much wider than long, the hinge-line is always the 

 greatest width of the shell, and though at times more pronouncedly 

 ' Avinged ' than at others, the angles of the shell are always acute. 

 The pedicle-valve is very slightly convex in the middle line, the 

 brachial valve flat or slightly concave with a bifid cardinal process. 



Shell punctate and ribs fasciculate as in 0. hivnantensis, but at 

 times rather coarser than in that species. 



This appears to be Strophomena silunana Davidson. 



YII. COMPAEISOX WITH OTHEE AeEAS. 



It is not very easy to compare the shelly faunas of one area 

 with those of another, since there is likely to he less uniformity in 

 the general physical conditions than in the case of those controlling 

 the distribution of the faunas of the deeper parts of the sea, and 

 hence faunas will he likely to show local jDeculiarities. Correlations 

 of this nature therefore can only be carried out on general lines. 



The Derfel Limestone contains a fauna which has not been, up 

 to the present, recorded at that horizon from any other "Welsh or 

 English deposits ; it does, however, show a noteworthy resemblance 

 to the fauna of corresponding Scottish deposits of that age, as the 

 list given on ]). 169 clearly indicates. On the whole, it compares 

 best with that of the Stinchar Limestone, though in some respects, 

 especially as regards the trilobites, it is closer to that of the Bal- 

 clatchie Beds ; it would appear therefore to contain a fauna of the 

 Scottish rather than the AA'elsh type, which is both interesting and 

 important on palseontological grounds. With regard to the Cara- 

 docian rocks of the Bala area, they may perhaps be said to 

 correspond in a general way to beds of similar age in the G-lyn- 

 Ceiriosc area described bv G-room & Lake,^ but with certain rather 

 strikino' differences. The Teirw Beds of that area mav be 

 paralleled roughly with the Allt-Ddu Mudstones (see lists, pp. 146- 

 47), and to some extent also the Bryn Beds have a fauna re- 

 semblins: that of the Gelli-ofrin Calcareous Ash : but a noteworthy 

 difference in the fauna seems to be the absence in the Bryn Beds 

 of Ortliis (^I^icoIeUci) fl'c^<9?z2<^', in variabh' highly characteristic of 

 the beds at Bala (see lists, pp. 147-48). This fossil does, however, 

 appear to be present in the higher shales when they are not faulted 



1 Q. J. G. S. vol. Ixiv (1908) p. 546. 



