6 NILS HJ. ODHNER, STUDIES ON RECENT CHAMIDAE. 



oblique cardinal groove, 4) a ligamental crest. Valve p has the following charac- 

 teristios: 1) an anterior striated cardinal tooth, 2) an oblique cardinal groove, 3) a 

 posterior cardinal tooth, 4) a ligamental crest. 



Valve a in the genus Chama is fixed and is the left one in the »normal» forms, 

 the right one in the »inverse» ones. In the genus Diceras, which was compared 

 vvith Chama by the author, it is always the left valve, in some cases the fixed one, 

 in others the free one. 



This idea that the hinge presents a uniform construction in the fixed valve a 

 of the Chamas, irrespective of whether it is right or left, has been adopted by Fischer 

 in his Manual de Conchyliologie (1887), by Cossmann & Peyrot (1911) 1 and by many 

 other authors, and from these sources it has been propagated in manuals and other 

 literature, and is repeatedly quoted by even the most modern authors. No attempts 

 at criticising and opposing it have been made. Only Bernard in his last paper (of 

 1897) on the hinge of Lamellibranchia tries to give proofs against Munier-Chalmas' 

 opinion about the symmetric valves. He remarks (p. 564) that in Eehinochama, 

 which he refers to the »normal» forms, »on voit distinctement, au-dessous de 3 a 

 (= the cardinal of the right valve) une dent rudimentaire 1, dont Téquivalent man- 

 que å la valve gauche des formes 'inverses'». This interpretation is, however, as will 

 be found belovv, not correct, and further it does not conclusively disprove Munier- 

 Chalmas' theory, because it has not revealed its principal defects. With this ex- 

 ception the theory of the symmetric valves has hitherto remained unrefuted. 



Tliere seems, however, to be one reason adducible in favour of the theory of 

 Munier-Chalmas, namely that it implies a case of the same kind of inversion as is 

 not uncommon among the Gastropoda. Here the so called sinistrorsity of chiefly 

 dextral ' forms (as well as dextrorsity of sinistral ones) appears partly as an occa- 

 sional teratologic phenomenon, partly as a fixed generic character in Clausilia, 

 Physa, Triphora and others. 2 How this anomaly has arisen is a problem not yet 

 solved, but it seems to be caused in the process of cleavage on the one hand and 

 in the sequent torsion of the visceral mäss on the other. Only among the Gastro- 

 pods, where a twisted shell is present, have cases of inverted organization been 

 observed. No example of it is known among externally symmetrical forms such 

 as Patella, nor have any been described among the Lamellibranchs, and the cause 

 of its non-existence here is certainly to be sought for in the permanent bilaterality 

 which prevails as a rule in these mollusca. Here] too torsions take place, it is 

 true, but these are restricted to the umbonal parts of the visceral mäss and are 

 effected bil äter ally. The result of a dextral torsion is a prosogyric, that of a sini- 

 stral one an opisthogyric ourving of the umbonal region. The possibility is, however, 

 conceivable that already in the early development of the embryo such an inversion of 

 the potencies of symmetry may take^place that it results in an inverse organization 

 of the full-grown animal, manifested by a dextral construction of the left-sided hinge 



1 The ' last-named authors, however, apply Berna»d's representing, thus odd numbers for the right and 

 even for the left valve. 



8 Cf. Dautzenberg li»l l. 



