DAVIDSON — SCOTTISH CAEDONIFEROUS BRACHIOPODA. 101 



XXIII.— Orthis Michelini * L'EveiUe. Plate i., figs. 7-10. 

 Terehratula Michelbii, L'Eveille, Mem. de la Soc. Geol. de Trance, vol. ii., p. 

 39, pi. ii, figs. 14-17, 1835. = Spirifera Jiliaria, Phillips' Geol. of York., vol. 

 ii, p. 220, pi. xi, fig. 3, 1836. 



This Orthis is either circular or longitudinally subtrigonal, the greatest 

 width being situated in the frontal half, while the front itself forms either a 

 gentle outAvard, or a slight inward curve. The hinge-Hne is abbreviated, and 

 at times does not attain one third the widtli of the shell ; both areas are con- 

 sequently very small ; but that of the ventral valve is the largest, and divided 

 by an open triangular fissure. The dorsal valve is moderately convex, with a 

 gentle depression towards the front, while the ventral one is very shallow, 

 slightly convex at the beak and along the'middle, from whence it becomes much 

 flattened towards the margins. The depressions of the ventral valve and slight 

 convexity of the dorsal one give to the shell a general depressed appearance, 

 which is one of the features by which it can be distinguished from 0. resupi- 

 nata. The beak of the ventral valve is small, slightly incurved, and projecting 

 but little beyond the level of that of the dorsal valve. The surface of the shell 

 is ornamented by numerous small radiating thread-like rounded striae, which 

 rapidly increase in number by numerous intercalations, while from all these 

 little ribs numerous small hair-Hke hoUow spines project, becoming more 

 closely packed towards the margins, so that, when alive, the whole shell must 

 have been invested with delicate spines, no where exceeding a quarter of an 

 inch in length. Prof, de Koninck who, in 1813, first noticed the spiny in- 

 vestment, was of opinion tliat the dorsal valve was nlone so provided, but I can 

 assure my distinguished friend that, although in the generality of specuneus 

 the spines on the dorsal valve were the most numerous, I possess several ex- 

 amples which prove beyond doubt that the ventral valve was also so provided, 

 although generally not to the extent seen on the dorsal one. In addition to 

 the spines, the whole surface of the shell is covered with minute punctures, 

 wliich are tlie external orifices of the tubuli or perforations which traverse the 

 entire thickness of the valve. It AviU not be necessary to describe in detail 

 the markings observable upon the interior surface of the valves, as I have 

 done so for 0. resiipinata, but a glance at the respective illustrations will suf- 

 fice to explain the differences in the species under description. These in the 

 ventral valve are evinced in the narroA^Tiess of the median ridge and less in- 

 clined slope of the oeclusor muscular impressions ; while in the dorsal valve 

 the space occupied by the oeclusor, devaricator, and ventral adjustor muscles 

 is Avider than in 0. resiipinata ; there is also a singular impression at the base 

 of the fissure marked N. in the figure, and which is with some uncertainty 

 referred to the pedicle muscle. 



0. Michelini is not a rare shell in Scotland. In Lanarkshire it occurs at 

 Langshaw Burn at three hundi'cd and seventy -five fathoms lower than the 

 "Ell Coal;" at Brockley and Middleholm, near Lesmahago ; Auchentibber, 

 Calderside, and Phillipshill, High Blantyre ; Capel Rig, East Kilbride ; and at 

 Bobroyston, north of Glasgow. In Renfrewshire, at Orchard-quarry, ThornHe- 

 bank ; Barrhead and Howood, near Paisley. In Ayrshire, at Rougliwood, 

 West Broadstone, and Treehorn, near Beith; Auchenskeigh, Dairy, Gold- 

 craig, and Monkredding, near Kdwimiing ; HaUerhirst, Stevenston ; Craigie, 

 near Kilmarnock, and Cessnock near Galston. In Stirlingshire it is found in 

 several stages. At Craigenglen, the Campsie main-limestone and Corrie Bum. 

 In Dumbartonshire, at Castlecary. In Eifeshire, at Charlestown and Limekilns, 

 above Queensferry. 



* AnomicB stricUcB of TJre, "History of Ruthergien and East Kilbride:" 1793. PI. xiv., 

 figs. 13 and 14. 



