THETIRONIA. 165 



Remarks. — The specimens from the Planer-kalk of Strehlen which were 

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

 (Zittel). 1 



Type. — The type came from the Chalk Marl of Middleham, but cannot now be 

 found. 



Distribution. — Upper Greensand (zone of Schloenbachia rostrata) of Devizes. 

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

 and Rinsrmer. 



"& j 



[Systematic position not determined.] 



Genus — Thetironia, F. Stoliczka, 1870. 



(' Palseont. Indica, Cret. Fauna S. India,' vol. iii, p. 158. Thetis, J. de C. Sowerby, ' Min. Conch.,' 

 vol. vi, 1826, p. 19. Non Thetis, Oken, 1815. Syn. Fimbriella, Stoliczka, op. cit„, p. 246.) 



The genus Thetironia [Thetis'] has been placed in the family Veneridae 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 quite unlike the pallial sinus of any lamellibranch, 2 

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

 suggested by S. P. Woodward 3 merely to strengthen the thin shell. Thetironia is 

 further distinguished from the Veneridas by its hinge, which is of quite a different 

 type (see Plate XXVI, figs. 10 b, 14). Stoliczka, in referring this genus to the 

 Veneridee and sub-family Dosiniinse, 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 pallial 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. 6). 



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



1 G. Midler, ' Mollusk. Untersen. v. Braunschweig u. Ilsede' (1898), p. 60. 



2 De Loriol compared it with Lucinopsis, but the differences between the pallial sinus of that genus 

 and the angular rib of Thetironia are considerable. 



3 'Manual of the Mollusca,' ed. 1 (1854), p. 319; ed. 3 (1875), p. 491. Woodward says, 

 " umbones strengthened inside by a posterior lamina." 



22 



