.10 NILS HJ. ODHNER, STUDIES ON RÉCENT CHAMIDAE. 



In Ch. gryphina some of the plications (r) are observable on the inferior margin of 

 the dental plate, at least in specimens where the teeth are distinctly developed; in 

 such a case even an accessory posterior tooth (x } ) may appear. In the right valve 

 the rugae are still present on the callus Y, but they exhibit no trace of a trans- 

 versal partition as in Echinochama. The posterior tooth y is merely an indistinct low 

 crest with or without crenelation. In Ch. cristella the tooth x of the left valve is 

 wholly reduced, the plicae r are absent or rather weak, but the margin of the dental 

 plate is elevated at this place; the callosity Y of the right valve is striated chiefly 

 on its upper side and the tooth y is more or less obscure. In Ch. cornucopia (fig. 

 3) the tooth X is less easily recognizable, because its apex is low and not distinctly 

 marked off from the shell; further it seems to pass into tooth x, which is strongly 

 developed and characterized by irregularly placed tubercles above and some stria- 

 tions on its lower (inner) side; these last-named are probable homologous with the 

 inferior plicae in Echinochama, which have become somewhat dislocated. Tooth x is, 

 as named, well-developed and distinctly tubercled on its inside. The projection Y 

 of the right valve is strong and bilaterally furrowed, and tooth y is observable as 

 an indistinctly tubercled riblet close beneath the ligamental line. 



Further reductions may take place (for example in Ch. exogyra, where the 

 tooth X gets a lower apex, seemingly combined with tooth x as in Ch. cornucopia, 

 and the callosity Y löses the furrows on its inner side and preserves them only on 

 the upper (outer) side. This dentition is a disappointingly perfect image of sym- 

 metry compared with that of the normal Chamas, but traces of tooth y, which has 

 no correspondence in those, prove the reduction of their hinge characteristics ac- 

 cording to the scheme shown above. 



From the t}^pe described just now the »normal» Chamas show marked diffe- 

 rences. Both Bernard and Anthony have chiefly studied such forms without com- 

 paring them with »inverse» shells. Their accounts diverge somewhat in details, as 

 will be shown below. Taking as our object the same type as Bernard (though 

 another species, Ch. macerophylla Chemnitz, fig. 4; ef. fig. 1), we find in it the fol- 

 lowing arrangement. 



In the left valve a large callosity projects on the inferior margin of the dental 

 plate; it is the tooth 2 of Bernard. It is striated only on its outer (upper) side 

 and in an oblicjuely transverse direction. No trace of an anterior tooth has been 

 found by the authors mentioned, nor does any exist in my specimens. Behind the 

 large tooth (2) there is a socket, and just under the ligament appear two small crests 

 with an interjacent furrow; the inferior of them is tooth 4 b of Bernard and tooth 

 4 of Anthony. The furrow and the exteriör crest have not been observed by 

 Bernard nor by Anthony. Lastly, behind the ligamental furrow, a knob-like pro- 

 jection represents a lateral tooth, designated by Bernard as LP II. 



In the right valve a long curved tooth-crest runs parallel to the superior shell 

 margin. This crest is rugated on its lower side and is divided into an anterior and 

 a posterior part b}^ a slight sinuosity. Bernard recognizes both parts, denoting 

 them by 3 a and 3 b, but Anthony thinks this a »precision trop grande» and gives 



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