SPIRIFERIDyE. 



99 



Spirifera (Cyrtia) exporrecta, Waldenherg. PI. IX, figs. 13 — 24. 



Anomites exporrectus, Wahlenherg. Nova Acta Regis Soc. Scient., vol. viii, p. 64, 

 No. 3, 1821. 



Cyrtia trapezoidalis, Hisinger. Bidrag. Sver. Geogn. Anteckn., vol. iv, p. 220, 

 tab. iv, fig. \,a,b, c, 1828. 



— EXPORRECTA, Daltnan. Kongl. Vet. Acad. Handl., p. 118, tab. iii, fig. 1, 



1828. 



— TRAPEZOIDALIS, Id. Ibid., p. 119, pi. iii, fig. 2, 1828. 



— — Hisinger. Lethsea Suecica, p. 73, tab. xxi, fig. \, a,b, 1837. 



— EXPORRECTA, Id. Ibid., pi. xxi, fig. 2, a, b,c, 1837. 

 Spirifer TRAPEZOIDALIS, Von Buck. Ueber Delthyris, &c., p. 41, 1837. 



— — Sow. Sil. Syst., pi. V, fig. 14, 1839. 



— — Bronn. Lethsea Geogn., tab. iii, fig. 3 ; Index Pal., p. 1183, 



1848. 



— — Davidson. Bull. Soc. Geol. France, 2nd ser., vol. v, p. 324, 



pi. iii, fig. 43, 1848. 



— — Barrande. Naturwiss. Abhandl., vol. ii, pi. xvi, fig. 1, 1848. 



— PLICATELLUS, var. EXPORRECTUS, Salter. Mem. Geol. Surv., vol. ii, pt. i, 



p. 382, 1848. 



— — var. TRAPEZOIDALIS, Id. Ibid., p. 382. 1848. 

 Spirifera (Cyetia) trapezoidalis, M'Coij. Brit. Pal. Foss., p. 196, 1852. 

 Spieifee TRAPEZOIDALIS, Salter. Siluria, 2nd edit., pi. xxi, fig. 3, 1859. 

 Spirifera exporrecta, Lindstrdm. Ofv. Vet. Akad. Forhandl., p. 358, 1860. 



Spec. Char. Trigonal ; valves convex ; hinge-line nearly or quite as wide as the 

 width of the shell ; cardinal angles rectangular, or obtusely rounded. Ventral valve 

 pyramidal, much more convex than the dorsal, with an angular median sinus, extending 

 from the extremity of the beak to the front. Beak straight or incurved, at times more or 

 less inclined backwards : area variable in its dimensions, usually large, triangular, some- 

 times almost equiangular; fissure narrow, entirely arched over by a convex deltidium, 

 which presents an elongated depression along its posterior portion, and terminated 

 anteriorly by a small oval foramen, which became cicatrised witTi age. Dorsal valve 

 semicircular ; sides gently convex or rounded, forming a deep subquadrate lobe at the 

 margin : the fold of moderate width and elevation, flattened along the middle. Surface of 

 both valves covered with numerous, fine, longitudinal, raised, thread-like striae, which 

 increase in number as they proceed towards the margin by bifurcation as well as by the 

 interpolation of additional strise. In the interior of the ventral valve the hinge-teeth are 

 supported by short shelly plates, w^hich diverge and extend from the extremity of the 

 beak, forming the fissure- walls, and occupying about one third of the length of the bottom 

 of the valve : between these there exists a slightly raised longitudinal ridge (but not 

 septum), which divides the muscular impressions. In the interior of the dorsal valve the 



