SPIRIFERID^. 



113 



Atkypa didyma, Sow. ' Silurian System,' pi. vi, fig. 4, 1839. 

 Terebratula canalis, Id. lb., pi. v, fig. 18, 1839. 



Atrypa didtjia (including T. canalis), Phillips and Salter. Mem. Geol. Surv., vol. ii, 



p. 277, 1848. 



Terebratula, — Dav. Bull. Soc. Ge'ol. Fr., 2nd sor., vol. v, p. 326, pi. vi, fig. 7, 

 1848. 



Spikifer didymus, Bronn. Index Pal., p. 11/6, 1848. 

 ^ Hemithyris diuyma, D'Orb. Prodrome, vol. i, p, 37, 1849. 



Atrypa canalis, Id. lb., p. 40, 1849. 

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



— UPSILON, Id. Id., p. 207, 1852. 

 Waldheimia ? canalis. Gray and Woodward. Cat. MoUusca Brit. Mus., part iv, 

 p. 77, 1853. 



Spirigeeina didyma, Schmidt. Archiv Naturk. Liv- Ehst- und Kurlands, vol. ii, 

 p. 210, 1858. 



Rhynchonella — Salter. ' Siluria,' 2nd edit., pi. xxii, fig. 15, 1859. 

 Spirigera — Lindstrdm. Ofv. K. Vet. Akad. Forhandl., p. 361, 1860. 



Spec. Char. More or less pentagonal, usually longer than wide ; posterior margins 

 converging into a small tapering incurved beak; front narrowed, and emarginate or 

 notched. Valves either regularly and moderately globose, or with a narrow mesial 

 sulcus, commencing in each valve at a short distance from the extremity of the beaks, 

 and extending to the front. No foramen visible. Interiorly, the spiral processes are 

 directed outAvards, and attached to the hinge-plate of the dorsal valve. Surface smooth, 

 marked here and there by more or less strongly indented concentric lines of growth. 

 Shell-structure impunctate. Two specimens measured — 

 Length 11, width 10, depth 7 lines. 

 10, „ 10, „ „ 



Ohs. This small species has, since 1S27, been referred to no less than nine different 

 genera ! Being possessed of spiral processes, it could not be classed with Terebratula, 

 Bhi/nchonella, or Hemithyris; and it appears to me that its affinities are more with 

 Meristella or Athijris than with any of the other genera to which it has been allotted. 

 Palaeontologists are also now generally of opinion that Terebratula canalis, Sow., must be 

 placed amongst the synonyms of the shell under description ; and Mr. Salter and myself 

 ascertained the matter beyond doubt by comparison of the original fragmentary valve 

 upon Avhich the so-termed T. canalis had been founded, the specimen being preserved 

 in the Museum of the Geological Society. Von Buch was in error in placing it amongst 

 the synonyms of Terebratula sacculus, these shells belonging to different genera ; and 

 therefore Bronn was right in stating that Terebratula didyma, Von Buch, was a different 

 shell from T. didyma of Dalman. Not being acquainted with the Terebratula corculum of 

 d'Eichwald, I cannot say whether Prof. Bronn was correct in referring it also to Dalman's 

 species. I am not quite certain that the shell figured by Barrande as Terebratula 

 canalis, Sow. (Silur. Brachiop. Bohmen, PI. XVI, fig. 13), really belongs to Sowerby's 



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