158 CEETACEOUS PELECYPODA 



ascending, nearly pointed at the end, resembling that of Gemma ; hinge with three 

 teeth in each valve, very thin and laminar, the two anterior in the right and the 

 two posterior in the left closely approximate, the former almost perpendicular, the 

 latter very oblique, they appear almost as one entirely divided tooth ; besides that 

 there is a single perpendicular anterior cardinal tooth in the left and an oblique 

 posterior, sometimes very slightly bifid, tooth in the right valve ; the ligament is 

 sub-external, in a deep groove, and rather strong externally. Type (7. papyracea, 

 Gray. I have already stated that doubts can be entertained as to the proper 

 classification of this genus in this place, but the animal is said to resemble Dosinia. 

 The shells, with the exception of the angular pallial sinus, have scarcely anything 

 in common with each other. 



H. and A. Adams quote six species from the Philippines and from Aus- 

 tralia; one is said to occur at Ceylon. I do not know the habitat of these. I 

 obtained two apparently new species near Calcutta in brackish water, but have not 

 as yet been successful in observing the animal. Both the shells are typical 

 Clementice, very thin : one greatly recalls Tcmysiplion or Glauconomya^ both of 

 which are also brackish, and have an animal like Anatina ; the other species 

 is more inflated and also occurs near Piiri in a salt water lake (Chilka) and in the 

 Irawadi Delta; possibly it is the species noted from Ceylon. As previously 

 observed, I think it more probable that the genus will be better classed in 

 the family Glauconomyid^ than in the Ves'euidm ; but unless the animals be 

 re-examined, it will be impossible to come to any definite conclusion on this point. 



18. Thetironia, Stol., 1870, f Thetis, Sow., 1826, Min. Conch., vi., p. 19, non 

 Thetys = Thetis, Cuv., 1808). Shell roundly oval, tumid, very thin, somewhat 

 inequilateral, beaks incurved and rather close to each other ; hinge apparently with 

 one median larger and two smaller cardinal teeth, pallial line acutely angular and 

 very deep, reaching almost to the beaks. Type Th. minor, Sow. 



I have not seen a single specimen belonging to this genus with the shell pre- 

 served, but it seems to have been very thin, and there can be little doubt that all 

 the external characters indicate a close approach to the recent dementia. The 

 hinge-teeth are only known from impressions, and even these have not as yet 

 been observed in a perfect state. The principal characteristic of the genus lies in 

 the remarkably deep, posteriorly, narrowly, and angularly bent pallial impression. 

 The species of Thetironia are as yet only known from cretaceous deposits. 



In looking over the various forms which have been referred to the dosiniin^, 

 we observed that there were scarcely any species known from palaeozoic beds; in the 

 lower mesozoic deposits shells of Dosinia - form occur, but they have not as yet been 

 sufficiently discriminated from Lucina, In the jurassics we have also only a few 

 doubtful Dosinice, but several species of JEriphyla; in the cretaceous Cyprimeria Si>nd 

 Thetironia, and very likely also Dosinia are added. Gemma and Cyclina are as 

 yet doubtful, but they certainly occur in the lower tertiaries, and from the miocene 

 time Dosinia greatly prevails over all the other genera, There are about 130 recent 



