14 APPENDIX. 



Page 9, Discina. 



Four species have been described under the generic appellation of Orbicula, which, as stated in the 

 general introduction, must be changed to that of Discina : Orbicula being a synonym for Crania. 



Discina Townshendi, Forbes. 

 Every exertion has been made to discover the exact age of the bed and locality from -which this 

 magnificent species or specimen was obtained ; in page 9, it was erroneously supposed from the Oxford 

 Clay, but I subsequently became convinced that its real age was that of the Lias, and marked it such in the 

 general table, p. 98. Mr. Walton was informed that the shell was obtained by Townshend from the lias of 

 Fretherne Cliff, on the banks of the Avon, near Newnham (Glocestershire), and the Rev. P. B. Brodie 

 states he has seen two other examples in a collection near that locality. Discina Babeana (D'Orbigny) 

 is likewise from the Lias, and has attained similar dimensions; and it is possible that both this and 

 D. Townshendi may require to be united, although all the French examples I have been able to examine 

 were very much more convex or inflated. 



Page 10, Discina refiexa, Sow. 



Mr. Moore has lately discovered a small oval Discina in the Upper Lias, near Ilminster, which I am 

 inclined to believe belongs to the present species ; it measures — length 4, width 3, depth 1 line. 



Page 12, Thecidium. 



To the list of ' British Jurassic Thecidia,' Mr. Moore will perhaps be able to add two or three additional 

 species : I shall at present only mention — 



Thecidium Deslongchampsii, Dav. Appendix, Plate a, figs. 6, 6" ic 



Thecidea Deslongchampsii, Dav. Annals and Mag. of Nat. Hist., vol, ix, 2d ser., 



p. 258, pi. xiii, figs. 6—9, 1852. 

 — — E. Deslong champs. Memoires de la Soc. Linneenne de 



Normandie, vol. ix, pi. xiii, fig. 26, 1853. 



Diagnosis. Shell inequivalve, longer than wide, irregularly oblong : fixed to submarine objects by the 

 flattened beak of the larger valve, moulding itself on the object to which it is attached, the remaining 

 portion of the valve is regularly convex, and deepest near the hinge : area short, wide, and irregular : 

 deltidium visible, but not sharply defined : dorsal valve as wide as long, operculiform, slightly convex and 

 flattened : surface smooth, interrupted only by a few concentric lines of growth : structure punctated : 

 hinge-line straight, valves articulating by means of two teeth in the larger valve and corresponding sockets 

 in the smaller one. In the interior of the dental valve, beneath the deltidium, three short lamellar 

 processes are seen to occupy about a fifth of the length of the shell ; the central one being the longest and 

 most elevated ; the other two, appearing at the base of the dental plates, converge gradually towards the 

 central one : a longitudinal rounded elevation extends also along the middle of the valve. In the interior 

 of the smaller valve, on either side of the sockets, a wide, thickened, raised, granulated margin surrounds 

 the shell, which, on reaching the middle of the front, directs itself longitudinally inwards under the 

 form of a narrow, acute elevated crest, and not much longer than half the length of the valve ; on 

 either side of this ridge and the inner edge of the margin, are seen two other slender rounded ridges, 

 covered with large granulations. Dimensions variable : length \\, width li, depth § line. 



Obs. The discovery of this shell in the Lept&na bed of Ilminster, is entirely due to Mr. Moore's 



