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No. S.— AN EARLY STAGE OF ACMAEA. 
BY EDWARD S. MORSE. 
The group of Mollusoa of which the common limjiet may be re- 
garded as the type, formerly inchided the most diverse genera. Even 
within fifty years Troschel inchided in this group such incongruous 
forms as Chiton and Dentahum., He suggested the ordinal name 
Docoglossa, referring to the curious plate-like dentition of its radula 
and this name is still retained for the order. We are indebted to Dr. 
W. H. Dall for first reducing the order to a rational association of 
genera by excluding not only Chiton and Dentalium, whose shells 
were so unlike the patelloid form, but also a number of genera whose 
shells were remarkably limpet-like, but whose soft parts were widely 
different, such as Siphonaria and Gadinia. 
The Docoglossa, as thus restricted by Dall, forms a very natural 
group, and, as we shall see, a very archaic one. The so called nauti- 
loid shell,^ which is so universally seen in the early stages of gastro- 
pods, even in forms w^hich in their later stages show no trace of a shell, 
as in nudibranchs and naked pulmonates, is a marked character in 
the ontogeny of the class. It will be interesting to inquire whether 
this nautiloid stage is eciually marked in all the genera of the Doco- 
glossa. Unfortunately we have but few contributions to the embryo- 
logical history of this group; indeed all that w^e know on this subject 
is contained in Professor William Patten's (9) valuable memoir on the 
embryology of Patella vulgata. In this work, owing to conditions 
which he could not control, he was unable to carry the embryo beyond 
the suggestion of a shell. Only at one time did he succeed in obtain- 
ing a few embryos with a normal beginning of a shell in which also an 
operculum appeared. "Other embryos which had been kept twice 
as long, or fourteen days, developed no shell at all, although the con- 
ditions were apparently the same in both cases." In those stages in 
1 I have used the conventional term "nautiloid shell" in this paper, though this early 
stage is by no means nautiloid in form. Dr. Robert T. Jackson has called attention to 
the obvious inaccuracy of calling the protoconch of a gastropod nautiloid. It is simply 
a coiled sliell with one or more whorls, the apex being at one side. It is possible that 
in Homalogyra and Caecum the protoconch may be nautiloid, as an examination of the 
nucleus in these genera seems to indicate a shell coiled in a plane as in their later stages. 
