THETIRONIA. 165 



Remarl's. — Tlie spt'ciniens from tlie Plant-r-kalk of Strelilen which were 

 referred to this species by Geiiiitz appear to be related to Mutiella coarctata 

 (Zittel).i 



Ti/pe. — The type came from the Chalk Marl of ]\liddleham, hnt cannot now be 

 found. 



Distrihution . — Upper Greensand (zone of Schlmnharlna rostrata) of Devizes. 

 Base of Chalk Marl of Titherlcigh and Chardstock. Chalk Marl of Middleham 

 and Rino'mer. 



[Systematic position not determined.] 

 Genus — TnETiKONiA, F. Stolicd-a, 1870. 



(' Palaeont. Indica, Cret. Fauna S. India,' vol. iii, p. 1-58. Thetis, J. do C. Sowerb_v, ' Min. Concli.,' 

 vol. vi, 182G, p. 19. Non Thetis, Oken, 1815. Svn. Fimbnella, Stoliczka, op. cit., p. 24G.) 



The genns Thetironia \^T]ictis~\ has been placed in the family Vencridte by 

 Deshayes, d'Orbigny, Chenn, Stoliczka, Zittel, Fischer, Dall, and other authors, on 

 account of the presence of the acutely angular line which has often been regarded 

 as a pallial sinus. This angular line appears as a deep groove on internal casts, 

 and must consequently have been a prominent rib on the interior of the shell. In 

 its position and rib-like form it is fpiite unlike the pallial sinus of any lamellibranch,- 

 and it seems to be a structure of an entirely different nature, probably serving, as was 

 suggested hj S. P. Woodward ^ merely to strengthen the thin shell. Thetironia is 

 fui-ther distinguished from the VeneridtB by its hinge, which is of quite a different 

 typo (see Plate XXVI, figs. 10/;, 14). Stoliczkii, in referring this genus to the 

 Veneridae and sub-family Dosiniinfe, says : " There can be little doubt 

 that all the external characters indicate a close appproach to the recent 

 dementia " ; the hinge, however, which seems to have been unknown to Stoliczka, 

 shows that this view of the relationship of Thetironia cannot be maintained. 



A concentric ridge seen near the ventral margin on some internal casts of 

 Thetironia has been regarded by some authors as evidence of a simple jiallial line ; 

 but the presence in some specimens of several similar ridges at different distances 

 from the margin suggests that they are really of the nature of growth-rings (see 

 Plate XXVI, fig. G). 



Thetironia was identified with Poromya by S. P. Woodward (1854) and by IT. 



1 G. Muller, 'Mollusk. Untersen. v. Braunschweig? u. Ilsede' (1808), p. GO. 



5 De Loriol compared it with Lucinopsis, hut the differences between the pallial sinus of that genus 

 and the angular rib of Thetironia are consideiwble. 



s 'Manual of the Mollnsca," ed. 1 (18.M), p. 310; od. 3 (IS?.--.), p. 401. Woodward says. 

 " umViones strengthened insiile by a posterior lamina. " 



