66 
rounded by two right cardinals, thus 3a and 3b, according to 
Bernard. An indistinct swelling of the hinge plate in front of 
tooth 3a represents a 5a. In the left valve a small 4a is visible 
in front of 2 and behind it a 4b, which may be well developed 
or indistinct in G. acrobeles. 
In Kidderia bicolor v. Mårtens (from S. Georgia; text-fig. 16) 
the conditions are the same: teeth 2 and 3a are the largest, 4a 
and 3b smaller. 
Bernard (1898) has stated the same type in the prodissoconch 
of Modiolarca pusilla. 
Neogaimardia rostellata Ta te represents a different arrange¬ 
ment. In a small specimen (1. 1 mm; text-fig. 17) a single tooth 
2 is present in the left valve, this fits in a Socket in front of tooth 
3a in the right valve; a small swelling before 3 represents a tooth 
1. In a somewhat larger stage (1. 1.5 mm; text-fig. 18) tooth 2 
has got a slight sinus on its inner side and comprises tooth 1, and 
this State of aflPairs becomes much more obvious in the adult shell 
(text-fig. 19) where also tooth 2 has acquired a hook-like shape 
round tooth 1. 4a (ai in the figure) has been removed farther 
towards front, and so has 5a in the right valve. 
The coexistence of both these types within the same family is 
certainly a sign of primitivity. So are also the features of the li¬ 
gament which is often very indistinctly limited and may vary be- 
tween external and internal position. 
Concerning the structure of the gilis this family exhibits a 
parallel to the Cyrenacea {Cyrenidae and Sphaeriidaé)\ also in this 
group we find simple as well as folded gilis and even a reduction 
of the posterior (or outer) demibranch, no doubt as a consequence 
of the incubatory development and its accumulative nursing of ju¬ 
venile characters respectively arresting of progressive modification. 
Thus the single gili in Neogaimardia is certainly to be compre- 
hended as a secondary apparition due to reduction, in the same 
way as the similar condition in the single-gilled subgenus Neopi- 
sidium of the freshwater mussels Pisidium (cf. Odhner 1923). 
To Neogaimardia also belongs Gaimardia tasmanica Bed dom s, 
according to specimens sent by the Australian Museum. 
Gaimardia acrobeles S u t e r. 
Auckland Island: 37, many shells, 1. 5.6. 
