SPIRIFERIDiE. 



123 



Tyrone, but Mr. Salter assumes that this is a mistake, and that A. compressa is strictly 

 Upper Silurian. On the Continent M. Barrande found the species in Upper Silurian 

 rocks near Prague. Dr. G. Lindstrom is not quite certain if it really occurs at Gothland. 



Athx-ris ? DEPRESSA, Sow. (sp.). PI. XII, figs. 11—15 ; and PI. XIII, fig. 6. 



Atrypa DEPRESSA, J. de C. Sow. Sil. Syst., pi. xiii, fig. 6, 1839. 

 Terebratula DEPRESSA, Bav. Bull. Soc. Geol. France, 2nd ser., vol. v, p. 327, 1848. 

 Atrypa — Phillips and Salter. Mem. Geol. Surv., vol. ii, part 1, p. 277, 



1848. 



Rhynchonella — Salter. Siluria, p. 544, pi. xxii, fig. 17, 1859. 

 Hemithyris — M'Coy. Brit. Pal. Foss., p. 201, 1852. 

 Spirigerina cordata, Lindstrom. Ofvers. K. Vet.-Akad. Forhandl, p. 363, pi. xii, 

 fig. 3, 1860. 



Sjjec. Char. Transversely obovate, widest anteriorly ; front nearly straight ; dorsal 

 valve varying in degree of convexity, but deeper than the opposite valve ; mesial fold of 

 moderate width, very slightly raised above the general convexity of the valve, and divided 

 by a central longitudinal sulcus ; the lateral portions of the valve sloping rapidly to the 

 margin, which in front is slightly incurved. On each of the lateral portions of the valve 

 there exist two or three obscurely marked ribs or undulations. Ventral valve moderately 

 convex, with a wide shallow sinus, extending from the extremity of the beak to the front, 

 along the middle of which runs a small longitudinal rib, two or three rounded ribs being 

 also present on the lateral portions of this valve. Beak pointed, small, and incurved, 

 with a narrow triangular opening under its extremity, but rarely visible, owing to the 

 incurvation of the beak. Lateral portions of the valve depressed towards the margin, 

 which is sharp and slightly sigmoid. Surface of valves marked with concentric lines of 

 growth. Shell-structure fibrous, not punctured. 

 Length 5, width 6.^, depth 3 lines. 



Obs. This little species differs much in ajjpearance under the varied conditions in 

 which it is found. Usually it occurs either in the state of imperfectly preserved internal 

 casts, or with only the inner layers of its shell still adhering to the internal mould. In 

 this condition it was described and represented by Mr. J. de C. Sowerby in the * Silurian 

 System and thus it appears, too, in PI. XII, fig. 13, of this Monograph. In this 

 state the shghtly raised ribs of the perfect shell are rarely observable, the casts being 

 obscurely marked with radiating lines, or by two or three furrows along the middle, as 

 described by Sowerby ; while the perfect shell (of which I have seen but two or three 

 examples) is as described in our diagnosis : carefully enlarged illustrations of it will be 

 found in PI. XIII, fig. 6. The internal cast of the dorsal valve (PI. XII, fig. 15 «) 

 shows well the four adductor or occlusor muscular scars, with a deep depression be- 

 tween the posterior pair ; and these are somewhat similar in their arrangement to those 



