338 BRITISH SILURIAN BRACHIOPODA. 



lower down than the " Middle Lingula-flags," where the genus is represented by L. 

 squamosa, a not well-determined species, it is true ; while L. pygmcea, another uncertain 

 species, has been found in the "Upper Lingula-flags." In the Arenig or Skiddaw 

 Group we have L. pdalon ; but it is chiefly in the " Llandeilo flags '' that the genus 

 begins to be represented by such shells as L. attenuata, L. brevis, L. granulata, L. 

 Mamsayi, and one or two others. In external ^\\di^Q Linyula petalon much resembles some 

 small examples of Z. cuneata and L. attenuata ; but it is always smaller and shorter than 

 either of those species. 



Position, and Locality. Lingula petalon was found for the first time, in 1864, by Mr. 

 Hicks, in the Upper and Lower Arenig or Skiddaw Group, at Whitesand Bay, near 

 St. David's, South Wales ; and subsequently in the same formation at Ramsay Island 

 and Tremanhire. 



Genus — Obolella, Billinys. 



In 1861 Mr. Billings proposed the genus Obolella, with the following diagnosis : — 

 " Shell ovate, circular or subquadrate, convex or plano-convex. Ventral valve with a 

 false area, which is sometimes minute, and usually grooved for the passage of the 

 peduncle. Dorsal valve either with or without an area. Muscular impressions, in the 

 ventral valve, four ; one pair in front of the beak, near the middle, or in the upper half 

 of the shell ; the other pair situated one on each side near the cardinal edge. Shell 

 calcareous; surface concentrically striated, sometimes with thin, expanded, lamellose 

 ridges. In general form these shells resemble Obolus, but the arrangement of the 

 muscular impressions is different. In Obolus the two central scars have their smaller 

 extremities directed downwards, and converging towards each other ; but in this genus 

 the arrangement is exactly the reverse." Such is Mr. Billings' diagnosis, but I fear it 

 will require some little modification in its details if it is to comprise 0. chromatica (the 

 type), 0. (?) polita, 0. desiderata, 0. sagittalis, 0. niaculata, 0. (?) Salteri, and several 

 other species. Unfortunately the American material in my possession is not sufficiently 

 complete to enable me to determine the point in question ; the internal cast of one of the 

 valves of 0. desiderata being the only interior I have been able to examine. The interior 

 of 0. polita will be found figured in ' The Sixteenth Annual Report of the Regents of the 

 University of the State of New York, for 1863,' and some complete illustrations of both 

 valves of 0. sagittalis are here given. Drawings of these last having been sent to Mr. 

 Billings, he wrote back — *' Your figures show the great muscular scars of Obolella, but 

 their proportions are quite different from those of 0. chromatica, 0. desiderata, and 0. 

 polita. In HalPs figures of 0. polita the muscular scars agree with those of my species, 

 as nearly 8>s two forms of the same genus usually do; but the two anterior scars are 





