16 NILSHJ. ODHNER, STUDIES ON RECENT CHAMIDAE. 



by a deep pit, a long posterior cardinal appears, homologous with that of Echino- 

 chama; further a faint trace of a lateral tooth behind the ligamental notch is per- 

 ceptible. 



Passing över to the right or fixed valve (fig. 8, right ) we observe in it a lengthened 

 crest-like element beneath the umbones, extending from the anterior muscular im- 

 pression to a large hole in the dental plate, limited behind by a narrow laminar 

 tooth just beneath the ligament. A tooth-like projection, lastly, can be seen just 

 before the posterior end of the ligamental furrow, and further there is a small trace 

 of a posterior lateral tooth behind the ligament. 



If we consider how teeth of opposite valves alternate, we shall find the following 

 conditions: When the valves are closed, the apex of the left or upper valve is situated 

 beneath and somewhat in front of that of the right valve. Hence it is evident that 

 the foremost crest of the left valve seizes in front of the anterior part of the 

 foremost crest of the right valve; the median tooth fits in the middle of this 

 crest, and the posterior one in the pit behind it and in front of the lameilar tooth 

 beneath the ligament. As the left median tooth is obviously homologous with the 

 median tooth of the young Echinochama, the teeth of the right valve, which surround 

 it, must be homologous with the right cardinals in the nepionic Chama ferruginea 

 just described. These last-named, consequently, are represented in the present form 

 by the front and the back parts of the crest. A closer examination of it also suggests 

 a division by means of a deeper median furrow into two separate projections, each 

 of them being in its turn furnished with an apical groove. 



It is important to state that the lateral teeth alternate, in the stage just de- 

 scribed, in the same manner as in the nepionic stage; thus the left lateral grasps on 

 the outside of the right one. This alternation is quite similar to that persisting in 

 Chama (ef. above). In the adult specimen of Echinochama, however, as we have 

 found in the preceding chapter, the alternation is reversed, so that there is, in the 

 left valve, a tooth clasping inside the right lateral. It is evident that the left tooth 

 in question cannot be homologous with the left lateral of the nepionic stage or with 

 LP II of Chama; it is, moreover, a new manifestation and represents a marginal 

 tooth (M), such as I have described in Trigonia and Unio (ef. Odhner 1918). This 

 originates in the form of a tickening of the dental plate, where a new counter- 

 balance to tooth 1 is required as a compensation for LP II which becomes obliterated 

 during development. 



A complete confirmation of the validity of the conditions described above con- 

 cerning the strueture and the development of the hinge was gained by me quite by 

 chance. On a piece of Cladocora caespitosa, a coral from the Mediterranean that 

 happened to come in my hands, I found some small attached individuals both of 

 Chama gryphoides and of Ch. gryphina. My idea of finding young specimens of 

 their free stage in the mud between the coral stocks was realized; the catch con- 

 sisted of one right and one left valve of the nepionic Ch. gryphina and some small 

 valves of Ch. gryphoides. The Identification of the valves was made possible by 

 comparison with the small attached individuals and is all the more certain as only 



