ORTHIDtE. 249 



one specimen," and Mr. Salter states that "a few strong fringe-like varices of growth 

 occur near the margin, giving the old shell an antiquated aspect," and that " at about half 

 way down the shell the ribs fork in most of the varieties, and in many specimens fork 

 again ; when this is the case the ribs become angular instead of rounded." In PI. XXXIV 

 I have given a selection of figures in which all the important features or characters 

 belonging to the species have been carefully represented, and a comparison of these with 

 those of 0. calligramma (PI. XXXV) will, I think, convince the student as to the necessity of 

 keeping them separate. "As justly remarked by Mr. Salter, 0. flabellulum bears a closer 

 resemblance to 0. porcata, M'Coy, an extremely near ally, but from which it can be 

 distinguished by its wider or coarser ribbing, as well as by several other characters. In 

 the extreme variety, 0. flabellulum, var. multifida, from Snowdon, the number of ribs is 

 greatly increased, but we are already prepared for such variations by the parallel case of 

 0. calligramma, and I might add of most species, for in none do we meet with the exact 

 same number of ribs on any individual. Beautifully preserved internal casts are often' 

 found, and it is from one of these in the Woodwardian Museum that I have made the 

 gutta-percha cast from which the enlarged figure (12 a), showing the interior of the 

 ventral valve with its vascular impressions, has been illustrated. 



Position and Locality. Orthis flabellulum is a common shell in the Caradoc or Bala 

 formation, and has been obtained from many localities in England, Wales, and Ireland. 

 Murchison found it in the east flank of the Caradoc at Gorton, Clunbury, and other places 

 in the true Caradoc Sandstone. It is extremely abundant at Boduan, Carnedd Dafydd, 

 Llyn-Idwal, Llyn-Ogwen, Bettws-y-coed, and Dolwyddelan, Caernarvonshire; at Cefn 

 Llyfnog, south-east of Cerrig-y-Druidion, Glyn Ceiriog, Denbighshire; Llanwddyn, 

 Meifod, north of Llangedwin, Llanfyllin, Montgomeryshire ; east, west, and south- 

 east of Bala Lake, Llanrhaidr, and Snowdon, Merionethshire (Ramsay and Salter), M'Coy 

 names also several more localities, such as Coniston, North Lancashire ; east of the 

 Berwyn Mountains ; Cader Dinmael, near Corwen, Denbighshire ; the Bala Limestone of 

 Applethwaite Common, Westmoreland, &c. 



In Ireland it is stated by M'Coy to be abundant in the grey calcareous slates of 

 Kilbride, Cong, County Galway; and in the schists and sandstone of Shanbaliymore, 

 Cong, County Galway ; and I have several fine internal and external casts from Grange 

 Hill, Chair of Kildare, sent to me by Mr. Joseph Wright, of Cork. 



In America Prof. Hall mentions it from Lockport, Rochester, Sweden, Wolcott, and 

 other places. 



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