260 Mr. T. Davidson on French Liasic Spirifers. 



than long ; attached by the whole surface of its larger valve to 

 submarine bodies; area triangular; deltidium well defined; 

 unattached valve slightly convex, but flattened ; surface smooth 

 and punctuated. 



Obs. I mentioned in 1851 that M. Tesson had met with a 

 very large liasic Thecidea closely resembling our Th. Bouchardii, 

 found by Mr. Moore in the Lias of Ilminster, but the dispropor- 

 tion in size between the specimens discovered at that period made 

 me uncertain as to the identity, of which I have now not the 

 smallest doubt, Mr. Moore having lately obtained in the same 

 locality several adult specimens of Th. Bouchardii, quite agreeing 

 with the French shell, and even slightly exceeding it in dimen- 

 sions. 



Thecidea Bouchardii was found by M. Tesson attached to a 

 Spirifer Tessoni, in the Lias of Fontaine-etoupe-four near Caen ; 

 it measures, length 2| lines, width 4 lines. 



Plate XIV. fig. 10. M. Tesson's specimen, natural size ; 10 a. enlarged 

 illustration. Fig. 11. Our English specimen, natural size; 11a. 

 the same, enlarged. Fig. 12. A very adult specimen, natural 

 size, found near Ilminster by Mr. Moore, attached to a Rhyncho- 

 nella serrata ; it measures, length 3, width 4 lines. 



Thecidea triangularis, D'Orb. PI. XIV. fig. 13. 



Thecidea triangularis, D'Orb. Prodrome, vol. i. p. 316 (1849) ; 

 Dav. Brit. Fossil Brach. part 3. p. 14. pi. 1. fig. 11, 12 

 (1851). 



This species is only here introduced on account of its being 

 found in England in the Lias and Inferior Oolite; while in 

 France it has only hitherto been discovered in the Great Oolite 

 of Ranville near Caen, where the shell is not very rare, attached 

 to shells and corals. The illustration fig. 13 is from a specimen 

 found in that locality. 



The three above-described Thecidea comprise all the species 

 of the genus with which I am acquainted at present from the 

 Jurassic formations of France. 



6. Monograph of French Liasic Spirifers. 



The French Liasic Spirifers are so remarkable in shape and 

 specific characters, some being new and others little known, that 

 in order to meet the wishes expressed by several palaeontologists, 

 I here beg to offer a short monograph accompanied by correct 

 illustrations of all the species known as occurring in that king- 

 dom. 



The genus Spirifer, established in 1818 by Sowerby, contains 

 many species, which may advantageously be divided into four 

 sections, from differences existing chiefly in the structure ob- 





