KTTNGL. SV. VET. AKADEMIENS HANDLINGAR. BAND 59. N:0 3. 9 



plate from the foot of the crest-tooth up to the foremost end of the socket. The 

 hindmost ridge is the largest of all. On the upper side of the pit a row of smaller 

 tubercles appears, which are wholly separated from and do not correspond in posi- 

 tion to the plicae on the inferior side. Behind the ligamental furrow, just beneath 

 its end, a knob-like projection represents a reduced lateral tooth. 



In the right valve the hinge shows a large anterior callosity extending from 

 the anterior muscular scar to the middle of the hinge plate and irregulary furrowed 

 on its upper and lower (inner) sides. The plicae on its inner side coalesce at the 

 inferior edge of the dental plate, and the anterior furrows are, before they terminate 

 at the lower edge, somewhat deepened, thus causing a partition of the callosity in 

 question to appear as a larger upper-hinder and a smaller under-frontal part. Behind 

 the features just described follows a broad groove, somewhat rugated at its bottom 

 and crossing the dental plate in an oblique direction from above to below. Behind 

 the groove and close beneath the ligament, a row of small denticles is to be ob- 

 served, running parallel to the ligamental furrow. Just below the inferior end of 

 the ligamental groove occurs a small socket; its somewhat callous upper margin re- 

 presents a reduced lateral tooth. 



Before we can identify the elements of this hinge, we must take into consi- 

 deration in what manner they alternate. When the valves are closed on each other, 

 the foremost feature of the hinge is the row of tubercles of the left valve situated 

 in front of the large socket. We will denote it, provisory, by x. As number 2 we 

 find the chief part of the large callosity of the right valve fitting in the socket of 

 the left one; this part will be marked as YA. The rugations below the socket in 

 the left valve we denote by r; they grasp under YA, alternating with its plica- 

 tions. Behind these rugations (r) fits a small posterior part ( Yp) of the right callo- 

 sity. The socket behind the right callosity receives the crest-tooth (X) of the left 

 valve, and the row of tubercles (y) behind the socket grasps on the outer side of 

 this crest-tooth. Lastly, the seeming lateral of the left valve (L) fits in the small 

 pit inside the vestigial right lateral (l). 



Schematically expressed, the alternation of the teeth of Echinochama may con- 

 sequently be illustrated as follows (the size of the letters corresponding to that of 

 the teeth): 



cardinals laterals 



left valve: x r X L 



right valve: YA Yp y l 



Quite the same elements of the hinge are recognizible in all the so-called »in- 

 verse» forms hitherto included in the genus Chama, for example Ch. gryphina and 

 Ch. cristella, and this circumstance proves that, contrary to Bernard's opinion, 

 Echinochama must be referred to the same group; though regular and equivalve this 

 species is also attached by its right valve. In the other »inverse» forms, however, 

 reductions may take place in one or other detail. In most of them the crest-like 

 tooth X of the left valve is recognized at the first glance by its projecting apex. 



K. Sv. Vet. Akad. Handl. Band 59. N:o 3. 2 



