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BRITISH DEVONIAN BRACHIOPODA. 



Localities. — The same as those already given for A. reticularis, and in addition to 

 which Prof. M'Coy adds Pridmouth, Menabilly, and Fowey in Cornwall, but from these 

 places I have not seen specimens. 



Atrypa desquamata, Sow. PL X, figs. 9 — 13; PI. XI, figs. 1 — 9. 



Atrypa desquamata, Sow. Trans. Geol. Soc, 2nd ser., vol. v, pi. lvi, figs. 19, 20. 



— — var. compressa. Ibid., figs. 21, 22. 



Spirigerina desquamata, M'Coy. British Pal. Foss., p. 378, 1852. 

 Atrypa — Murchison. Siluria, 2nd ed., p. 298, fig. 5, 1859. 



Spec. Char. Shell of a transverse oval shape, sometimes somewhat subquadrate, 

 straight or rounded in front, or broadly raised at the edge of the ventral valve without 

 elevating the surface of the opposite one. When young the shell is much compressed, 

 with almost equally deep valves, but with age the ventral valve becomes uniformly convex, 

 and even at times ventricose, especially along the middle, while the dorsal valve is convex, 

 but less so than the opposite one, and slightly concave towards the front and lateral 

 margins of the valve. The front margin of this valve forms an upward curve, while that 

 of the dorsal valve is concave. Beak more or less produced and angular, but slightly 

 incurved ; foramen circular, close under the pointed beak, and completed or almost entirely 

 surrounded by a wide deltidium in two pieces, and thus separated more or less from the 

 hinge-line, a flattened space or false area existing between the beak-ridges and the hinge- 

 line. Surface of both valves ornamented with numerous small rounded sub-equal 

 radiating ribs, with interspaces of equal width, the ribs augmenting in number at variable 

 distances from the beaks by the means of bifurcated or interpolated ribs. The surface is 

 also traversed by numerous concentric lines or slightly projecting laminas. Proportions 

 variable, a large specimen measured 2 inches 7 lines in length, 2 inches 4 lines in 

 width, and 1 inch 2 lines in depth ; but the generality of specimens are smaller. 



thick rounded ribs, with comparatively few branches, and distant, strong, concentric scales of growth, passes 

 by the most insensible gradations into that species, when a large series either from the Eifel or Devon is 

 examined. The most marked types of this peculiar form are nearly orbicular ; the length and breadth 

 equal ; length about one inch, the depth half the length ; the entering valve moderately convex along the 

 middle, raised into a small rounded sinus at the margin, receiving valve slightly convex at. the sides and 

 near the beak, becoming depressed in the middle towards the margin ; about sixteen thick, rounded, 

 radiating ribs, two or three only of which dichotomise, (occupying a space of two lines at six lines from the 

 beak, and the concentric laminae nearly a line apart at the same distance). This highly typical form of the 

 var. aspera is common in the Devonian limestone of Plymouth, and not uncommon in the Eifel ; but with 

 these in both localities occur specimens in which the depth in proportion to the length is -/q^ ; the ridges 

 become gradually smaller, more numerous, and more often branched, and the concentric laminae of growth 

 become more crowded, so that no character can be assigned, so far as my observations go, which would 

 serve to separate specifically the intermediate varieties." 



