24 Mr. W. H. Dall on certain 
little by little information has been coming in which fills the 
gap then known to exist in our knowledge of the order. It 
is now possible to review more understandingly the relations 
of the dentition of the different groups. It would seem at 
first sight as if the dentition of Lepeta and Lepetella differed 
very widely ; but more reflection diminishes the apparent di- 
vergences. 
It may be suggested that in Lepeta ceca the large rha- 
chidian tooth really represents a consolidation of the six 
laterals characteristic of Acmea, which is supported by the 
fact that G. O. Sars * figures the lateral cusps of the rhachi- 
dian tooth in Pilidium fulvum as accessory rather than in- 
herent parts of that tooth, a view (I find on reference to them) 
supported in part by my own original drawings, and a condi- 
tion which, though not universal or necessary, may yet be 
characteristic of some stages of the development of the indi- 
vidual or of the radula, or perhaps of some individuals merely, 
while in others the consolidation goes so far that the sutures 
(as in the bony structures of higher animals) are obliterated. 
In that case the rhachidian tooth of Lepetella would represent 
the consolidation of the two inner laterals merely, if the num- 
ber six be taken as typical, which, from its universality 
elsewhere in the order, we may reasonably assume to be the 
case. This is the typical number in the Tenioglossa, to 
which (as Professor Verrill indicates in his description) the 
radula of Lepetel/a is in some respects analogous, though the 
Tzenioglossa have no uncini. In the same way, as has before 
been pointed out, the single large dentate laterals of Pectino- 
donta may represent, in the other division of the order, each 
a consolidation of the three typical laterals of Acmea. 
The name Onychoglossa has been used by G. O. Sars (1878) 
to denominate the same group and as indicative of the same 
characters as those possessed by the Docoglossa (Troschel, 
1861), as revised by me eight years previously. I do not see 
any especial gain which might result to science from substi- 
tuting the newer for the older name. 
The relations of the groups may be expressed somewhat as 
follows :— . 
Order DOCOGLOSSA. 
Shell wholly external, dish-shaped, with apex anteriorly 
directed ; animal with two short tentacles, a non-extensible 
muzzle; branchie external or none; renal and anal aper- 
tures situated above the neck, between body and mantle- 
* Moll. Reg. Arct. Norveg. tab. ii. fig, 12. 
