KUNGL. SV. VET. AKADEMIENS HANDLINGAR. BAND 59. N:0 3. 11 



the whole crest the single number 3. A small narrow ridge-like tooth above the 

 large one has been overlooked by the authors mentioned; here, in accordance with 

 Bernard's method of notation, we number it 5 b. Lastly, below the lower end 

 of the ligamental groove, a small knob arises as a vestigial lateral tooth (LP I of 

 Bernard). 



As an important feature, though inconsiderable in appearance, we note finally 

 a small thickening of the dental plate margin situated in the right val ve beneath 

 3 a. Bernard (1895, fig. 22) has recognized in it a tooth 1. 



An examination of the alternation of the hinge elements shows the following 

 conditions: As the central tooth in the hinge, when the valves are closed, we find, 

 in accordance with Bernard, tooth 2 of the left valve, accompanied bj r a small 

 vestigial or rudimentary 1 of the right one; outside 2 and concentric with it runs 

 the curved 3 a + 3 b. Behind 3 b fits 4 b (left valve), outside of it 5 b (right valve), 

 and lastly the exteriör ridge of the left valve, which may be designated as 6 b. 

 The lateral of the left valve (LP II) is received outside the right one (LP I). A sche- 

 matic exposition of these conditions is given in the following survey: 



cardinals laterals 



left valve : 2 4 b G b LP II 



right valve : 3 a 1 3 b 5 b LP I 



Modifications of this scheme are to be found in other species of »normal» 

 Chamas. Thus in Ch. sinuosa the teeth 3 a and 3 b are quite distinctly separated 

 (more obviously than in Ch. macerophylla), and tooth 2 rises from a more triangulär 

 foot, but the exteriör crest-teeth of both valves (4 b, 5 b and 6 b) are indistinct, be- 

 cause the hinge plate above tooth 3 b is narrower than half this tooth. In Ch. 

 florida, on the other hand, 3 distinct crests are visible above tooth 3 b (they may 

 thus be designated as 5 b, 7 b and 9 b), and the plate bearing them is quite as broad 

 as tooth 3 b. In Ch. f rondosa tooth 3 b has been entirely suppressed and only 3 a 

 is present. In. Ch. iostoma, on the contrary, the only right tooth is 3 b (except an 

 almost indistinct knob-like 5 b), and the anterior 3 a is wholly reduced (only some 

 granulations mark its place). This extreme case of reduction vividly recalls the 

 conditions prevailing in the »inverse» Chama cornucopia, if the valves are compared 

 symmetrically; and the similarity is all the more striking because the colour as well 

 as the solid consistency of the shells agree. Nevertheless the two species must be 

 kept apart and allotted to different groups of Chamas, according to the exposition 

 of the hinge characteristics made in the above dissertation. 



It now remains for us to make a close comparison of the two types of Chamas 

 and to homologize the hinge elements in both the groups. This comparison will 

 start out from the two different points of view discussed in the present paper, that 

 is from my own opinion and from that which has prevailed hitherto and derived 

 from Munier-Chalmas and Bernard. 



In both cases, however, the comparison is based on the results obtained in 



