1G6 CRETACEOUS LAMELLIBRANCHIA. 



and A. Adams (1858). Deshayes (1858), and Pictet and Campiche (18G5), 

 though considering them allied forms, did not regard them as generically 

 identical. The latter authors had little confidence in the suggested relationship of 

 Thetironia to Venus, but in the absence of specimens furnishing decisive evidence 

 they left the former genus in the family Veneridas. In its external form and thin 

 shell Thetironia shows some resemblance to Poromya, but the conspicuous internal 

 ligament found in the latter is absent in the former. 



In a systematic list of Mollusca, J. E. Gray ' placed Thetironia in the Lucinidse, 

 but gave no reasons for assigning it to that family. Although elongate markings 

 do occur in the interior of some species of Lucina, they show but little resemblance 

 to the angular rib of Thetironia. Further, the hinge is unlike that typical of Lucina ; 

 for although in some few species (e. cj. L. gibba, L. pennsylvanica) in which the 

 umbones have a considerable anterior curvature, the positions of the cardinal teeth 

 become somewhat similar to those in Thetironia, yet this is clearly a secondary 

 character due to torsion and cannot be taken as evidence of any affinity between 

 Lucina and Thetironia. 



The hinge in some of the Corbidro, such as Sphxriola, Qonodon, and Mutiella 

 resembles that of Thetironia, but is far stouter, and the shell is much thicker. The 

 hinge in Unicardium also is somewhat similar to that of Thetironia , since lateral 

 teeth are absent, but the cardinals are less developed. Unicardium, however, 

 differs from Thetironia in the character of its ornamentation and in the form of 

 the shell. 



In both form and position the teeth of Thetironia show a striking resemblance 

 to the cardinal teeth of the Cardiidce (especially to Protocardia), and the hinge- 

 margin in front of the umbo expands in a similar way ; some further resemblance 

 is seen in the position and prominence of the external ligament, and in the general 

 form of the shell, which is similar to that of the nearly smooth Gardium (Serrijics) 

 grcenlandicum, Chemnitz. 3 The greater development of ornamentation on the 

 posterior part than on the remainder of the shell in Thetironia is also suggestive 

 of some forms of Protocardia. The position of the cardinal teeth in the Cardiidae 

 is practically constant and is a character of systematic importance ; so that the 

 close resemblance between these teeth in Thetironia and in the Cardiidae seems to 

 indicate a real relationship. Opposed to this, however, is the absence of lateral 

 teeth 3 in Thetironia, ; but although these teeth are usually found in the Cardiida^, 



1 ' Synopsis Brit. Mus.,' ed. 44 (1842), p. 91. 



2 Although d'Orbigny placed Thetironia in the Veneridse, he recognised that in some respects it 

 clearly resembles Cardium. See ' Pal. Franc. Terr. Oct.,' vol. iii (1846), p. 451. 



3 In the ' Additions and Corrections ' to his Monograph Stoliczka states that in Thetironia igndbilia 

 there are two cardinals and a minute posterior lateral in the right valve, and three cardinals in the 

 left valve. ' Taljcout. Indica, Cret. Fauna S. Indica ' (1871), p. 485. 



