302 CEETACEOUS PELECYPODA 



less parallel to tlie liinge margin, as in tlie South American 3Ioyiocond!jlce(e. But 

 the hinge-tooth of the right valve fits into an emargination situated in front of 

 the beak of the left valve, while the tooth itself of this latter valve is below 

 the beak.* In ijXiWdil Monocondylaa, as characterized by d'Orbigny, the tooth 

 of the right valve catches behind that of the left, but d'Orbigny adds, the 

 reverse is occasionally the case. I have unfortunately no series of South American 

 3Iouoconclijlcece to compare, but if this be really the case, I cannot see in which 

 characters Pseudodon-\ should differ from MonocondylcBa, except perhaps that the 

 Burmese and Malayan species composing it are more of a trapezoid than rounded 

 shape, and are more compressed than the American forms. 



5 b. Eor another Burmese species, described by Anthony in Am. Journ. 

 Conch., i, p. 205, as IIoiioc. crebrish'iata, Conrad proposed (ibid. p. 233) the 

 name Trigonodou. The hinge of this species has in the right valve an erect 

 almost vertically or obliquely elongated tooth fitting into a divided tooth of the 

 left valve, the posterior portion of the left tooth being much lai'ger than the 

 anterior. The hinge is perfectly the same as in Monoc. (or PseudodonJ Uuphratica, 

 Bourg., from Syria and America, and I cannot perceive any difference between 

 it and Margariluna. I have also examined the animal of the so-called Monoc. 

 crebristriata, and I find there is no difference between it and the last genus, as I 

 had already occasion to state. 



5 c. Eor another species again, Monoc. Mard'mensis, Lea, from the River 

 Tigris, Conrad pro j)osed, 1865, the generic navae JLeguminaia (Am. Joiu'n. Conch., i, 

 p. 283). Conrad says that it " has an outline approaching Mcirg. margaritifera, 

 being medially contracted and of an oblong and leguminous shape. The cardinal 

 tooth is pyramidal and recurved, wholly unlike the transverse compressed tooth of 

 JPseudodon." I received a specimen from the Biver Tigris through Mr. G. Nevill, 

 and this well agrees with Mr. Conrad's account. In another specimen in 

 Mr. NeviU's collection from the same locality the tooth is somewhat less prominent, 

 and in both extremely like, or I should rather say identical Avith, that of 

 3Iargarita7ia. 



Large series of all these shells in different stages of growth must be examined, 

 for there is no doubt that they gradually pass one into the other, and connect 

 Margaritana with Vuio. Certainly nothing can be more unnatural than creating 

 new generic groups upon the examination of single shells, — particularly among 

 such most variable forms as the fresh-water Unionid/e are known to be, — and then 

 attempting to assimilate the various species to the imaginary characteristics. It is 

 just the contrary way, we have to take in a natural classification. 



6. Gonidea, Conrad, 1857, (Proc. Acad. Nat. Sc, Phil., p. 105). Shell 

 " elongated, hinge furnished with a short, obtuse cardinal tooth, fitting into a 

 corresponding depression in the cardinal plate; tooth obsolete in' the left valve ; 



* Conrad's account (Am. Journ. Conch., i, p. 232,) is unintellif,'ilile on tliis j)oiMt. 



t Some of tlic old fossil-s)iecies cxtcraally very closely resemble this type of UslONID^, such species, for instance, 

 as the Woaldcn Unio JUai^ii, Dunk., Wealden-Bildung, 1846, p. 2S, pi. xi, figs. 1-3. 



