MORSE: EARLY STAGE OF ACMAEA. 319 
primitive kind of eve in the ^Nlolluscan series. They are found one on 
each cephahc tentacle and are simply minute open pits or dei)ressions 
of the epidermis, the epidermic cells lining them being pigmented and 
connected with nerves." 
Dr. Dall (1) in speaking of the Docoglossa as a group says the 
various forms manifest what may be termed a peculiar persistency of 
immaturity when compared with other groups of gastropods. Pel- 
seneer regards the Docoglossa and Rhipidoglossa as the most archaic 
of all gastropods. 
We have shown the primitive character of the embryo shell of A. 
testudinalis and have referred to the work and opinions of others 
testifying to the low character of the group; there are, however, a 
number of curious features associated with the Docoglossa that indicate 
its peculiar character. We find, for example, that the radula is 
markedly different from that of other gastropods, not only in the 
peculiar formula of its dentition but in its curious dark brown color. 
The radula of Acmaea is widely different from that of Patella vidcjata. 
While the egg of gastropods usually has a yellow yolk, Patten found 
that the egg of P. vidgata was bluish green. Dr. E. Ray Lankester 
discovered that the nephridia were paired in Patella and both were 
functional though one was larger than the other. 
It is interesting to observe that a diagrammatic view of Acmaea 
resembles somewhat closely the various figures given by authors of a 
gastropod archaetype. 
In considering these low and archaic characters of the Docoglossa 
it is interesting to inquire into the past geological history of the group. 
We find, in accordance with what might have been expected, that the 
earliest trace of molluscan life in the past is a group of shells in the 
Cambrian known as Capulids from their resemblance to a liberty-cap. 
These shells have been regarded as allied to Acmaea. Hall described 
a species from the Silurian which he named Palaeacmaea. The genus 
Scenella from the Cambrian, described by Billings, was regarded by 
him as belonging to the family Acmaeidae. Lindstrom describes the 
genus Tryblidium from these low geological horizons as related to 
Acmaea. Dr. Charles D. Walcott (11) in his second contribution to 
the studies of the Cambrian fauna of the United States gives a figure 
of Stenotheca rugosa (Fig. 10) which bears a remarkable resemblance 
to an early stage of A. testudinalis, as may be seen by comparison 
with Fig. 6. It will be noticed that the apex is turned to the right in 
