412 Miscellaneous. 
The Phylogeny of the Docoglossa. 
By W. H. Datt. 
In his concluding fasciculi, contributed to complete Troschel’s 
classical ‘Gebiss der Schnecken,’ Dr. Johannes Thiele dissents very 
emphatically from some suggestions of mine in regard to the deriva- 
tion of the true limpets, made many years ago. At that time it 
appeared to me that the Lepetidse might represent the stem, some- 
what degenerated, from which the Docoglossa were derived. While 
IT attach, even in the present state of our knowledge, comparatively 
little importance to speculations of this kind, which can only be 
placed on a firm footing by extended embryological researches, it 
still seems to me that there is a solid basis for the hypothesis which 
I then suggested. 
There can be little doubt that the early type of Gastropod gill was 
situated much as in /%ssurella, on the *‘ back of the neck” behind 
the head, and that it was constituted of a stem with lateral lamellae. 
Originally paired and symmetrical, by circumstances incident to 
erowth and torsion one gill of the pair has in most cases become 
aborted, though its ‘‘ smelling organ” frequently remains, as in the 
limpets. There is also no doubt whatever that the protolimpet was 
derived from a form having a spiral shell. I have shown that Pro- 
puidium by its dentition is closely allied to Lepeta. Now Propi- 
lidium is said to have two gills, but certainly has at least one, of the 
type of Acmea. It retains a spiral nucleus through life, though it 
is partly cut off by asmall septum, whichis never completed. Other 
Lepetide also show a spiral nucleus when very young, but it is cut 
off completely and lost later. These other, mostly deep- or cold-water 
forms, have lost their gills and eyes by degeneracy, and the principal 
teeth of the radula show a tendency to become cemented together, 
while in Propilidium they are more or less isolated. Now in the 
Acmeeide and Patellide the nucleus is limpet-shaped from the 
beginning; the uncinal teeth (well developed in Lepeta) are 
degenerate and often lost in the Acmeas, but appear again in the 
Patellas, not, however, with the individuality and completely chitin- 
ous nature which is found in the corresponding teeth of Lepetide. 
We find therefore in Lepetidee the greatest number of archaic cha- 
racters (somewhat masked by degeneration of other organs) which 
remain in any of the three groups, and, whether most ancient or not, 
so far as these characters go the Lepetide are nearest to the proto- 
limpet. 
In my work on the ‘ Blake’ mollusks (1. p. 436) I said that Acmzidee 
of all the groups of Docoglossa is the most typical; that is, within 
the limits of that family are found assembled, sometimes in one and 
the same animal, the greatest number of organs which, taken singly, 
are characteristic of Docoglossa. This is strictly true; but Dr. Thiele 
