THE 



GEOLOGICAL MAGAZINE. 



No. III.— OCTOBER, 1868. 



I. — On the Genus Tbimebella, Billings.^ 

 By Dr. Gustav Lindstrom, of the University of Wisby, in the Island of Gotland. 



[PLATE XX.] 



MANY beds of limestone, belonging to the middle division of the 

 Upper Silurian formation of the Island of Gotland, consist 

 almost entirely of the fragments of a very large and peculiarly - 

 shaped Brachiopodous shell. It may be regarded as one of the most 

 characteristic fossils of these Upper Silurian rocks. Perfect valves, 

 giving a satisfactory view of its structure have never been found. 

 When Mr. Davidson, with his usual kindness, sent me copies of the 

 figures and descriptions given by Mr. Billings,^ of the new genus 

 Trimerella, found in Canada, I had no hesitation in referring both 

 the Canadian and the Gothlandic shells to the same genus. The 

 materials obtained by Mr. Billings seem to have been more incom- 

 plete than mine ; I have, therefore, been able to make some additions 

 to the description given by him. 



The greatest peculiarity consists in two siphons or tubes, that 

 penetrate the shell along the median axis of the valves, or on both 

 sides of it. These siphons, by degrees, taper off, and cease in the 

 vicinity of the apex of the valves ; their openings are of an ovate 

 oblique form on the interior surface of the valves, and almost in the 

 centre. An elevated shield, hiding the continuation of the siphons 

 in its interior, is formed by the concentric shell-layers that envelop 

 the siphons (Plate XX., Fig. 6), and, on the surface, by the 

 mantle and other soft parts. This median elevation is smooth, 

 having no impressions of muscular parts, and is deeply concave 

 along the median axis. The lateral walls of both siphons are 

 contiguous to the median axis of the valve, and continue as a straight 

 ridge for a considerable distance down towards the inferior margin 

 of the valve. The soft parts that secreted the concentric layers of 

 the siphons, by degrees moved downward during the growth of the 

 animal, filling the place they once occupied with shelly matter. 

 Thus we find the apices of the siphons are generally filled with 



' Translated and communicated by the Author from the " Ofversigt af KongL 

 Vetenskaps-Akademiens Forhandlingar," 1867. No. 5. pp. 253-257. 

 2 Billings, Geol. Survey of Canada, Pal. Foss., Vol. I., p. 167. 



VOL. v.— NO. LII. 29 



