56 Nils Hj. Odhner. 
radula distique présenterait un état plus primitif que la radula 
polystique<. PRUVOT supports his opinion by a chain of argument 
accompanied by diagrams illustrating the probable evolution of the. 
distichous radula into the polystichous, and arrives at the conclusion 
that »on voit que le type le plus primitif est encore celui de Lepi- 
domenia, ou il existe deux séries de forts crochets dentelés« etc. 
(ic p. IM): 
By adopting this theory, which has also been accepted by 
NIERSTRASZ (1908), one is enabled to explain the types of the 
radulae in far more natural manner than would be the case if a 
regressive development is assumed. In essentials this theory seems 
to be borne out by the most facts, though there may be some excep- 
tions. Thus for instance Dorymenia sarsi has a polystichous radula 
in connection with supra-radular separate openings of the salivary 
glands. In this case, however, it must be borne in mind that the 
radula still shows a distichous feature in its deep median groove. 
It undoubtedly represents a type which may be assumed as origina- 
ted from the distichous radula by a confusion of the basal mem- 
branes in the median line and the expansion of the primary den- 
ticles into larger isolated teeth. 
Thus it is evident that the polystichous radula may arise in- 
dependently of the position of the salivary ducts, a fact also pointed 
out by Nierstrasz 1902 and 1905. On the other hand I do not 
know of any instance where a distichous radula exists together 
with a sub-radular single salivary opening (cf. NIERSTRASZ 1905). 
Thus, on the whole, Pruvot’s view must be endorsed, and the 
distichous type may ta considered to be the pe one (cf. also 
NIERSTRASZ 1908). 
Also the monoserial radula of Alexandromenia may easily be 
deduced from the distichous type. In A. crassa the denticles ate 
still present, in A. valida they disappear, each end of the tooth 
plate still bearing a long processus, and in Å. agassizi the latter 
too are reduced, the teeth being nothing but the primary basal 
plates with erect margins. The position of the salivary ducts in 
this case too corroborates PRuvoT's hypothesis. 
Only on the above assumption is the radula of Simrothieila 
comprehensible. If it is compared with that of Kruppomenia, its 
development can easily be conceived: we find in Kruppomenia a 
distichous radula; in its posterior end the teeth consist of a basal 
plate (produced by cylindric cells with prolonged nuclei) carrying 
