PLATE XX. 



Trigonia {continued). 



Figs. 



1, 2. T. scapha, Agassiz. (P. 7^3.) 



1. Lower Greensand (probably Snettisham ironstone nodules), West Norfolk. The 



tji^e of T. hnnstantonensis, Seeley. Woodwai'dian Museum. Eight valve. 



2. Snettisham Clay, one third of a mile south-west of Snettisham Chui-ch. Museum 



of Practical Geology. Portion of left valve. Drawn from a wax cast of an 

 external mould. 



Genus — Myoconcha, Sowerhij. 



M. cretacea, d'Orb. Chloritic Marl, Maiden Bradley. Woodwardian Museum. 

 a, right valve ; h, ornamentation of same X 3. (P. 114.) 



Genus — Spondylus, Linnaeus. 



4. ;S'. Roemeri, Desh. Perna-hed, Atherfield. Woodwardian Museum, a, left 

 valve (umbo sliglitly restored) ; b, outline from tlie posterior end ; c, right 

 valve ; d, ornamentation of left valve X 3. (P. 116.) 



5 — 11. S. gihhosns, d'Orb. Cambridge Grreensand (derived from the Upper Gault), 

 Cambridge. Woodwardian Museum. (P. 117.) 



5, 6 a. Left valves ; 6 h, anterior outline ; 6 c, ornament of left valve x 2. 

 7 0. Left valve ; h, posterior of both valves ; c, right valve. 

 8a. Left valve; h, part of same x 2. 

 9 a. Left valve ; h, anterior outline. 



10. Left valve. 



11. Posterior outline of a specimen with talon-like right valve. 



