MORSE- EARLY STAGE OF ACMAEA. 321 
SceneUa rarians from the Stephen Shale, Middle Cambrian of British 
Columbia and in some of the specimens faint traces of radiating lines 
may be detected. These lines are interrupted by faint lines of growth 
which give a reticulated appearance to the surface. This feature is 
characteristic of the young of .1. tcsludinalis and is strongly marked in 
the young of .1. alveus. This feature begins immediately on the for- 
mation of the permanent shell and continues to matvirity. 
The difference between the ancient and the modern shell is that the 
former retains at a much later stage the rudimentary embryo shell, 
while in the modern representative the nucleus is lost by decollation 
at a very early stage leaving the peculiar cicatrix already described. 
Furthermore in the Cambrian form Stenotheca, the apex remains near 
the posterior end throughout life, while in the modern form the apex 
is at the posterior end in the earliest stages only. Thus in the early 
Cambrian forms a very close resemblance is seen to the very earliest 
shell stage of the modern Acmaea. 
If these resemblances hold true we have another demonstration of 
the recapitulation theory, or the law of morphogenesis so ably pre- 
sented by Hyatt in his memoir on the Arietidae, with his subsequent 
nomenclature of successive stages, and the later contributions by 
Beecher, Jackson, and Schuchert in MoUusca and Brachiopoda. 
In this paper I have dealt with Acmaea testudinaUs and A. alveus 
as distinct species. At the outset I began the work solely for the pur- 
pose of determining the specific value of Acmaea alveus and the 
propriety of its separation from A . testudinaUs. By the earlier students 
of the subject the two species were regarded as distinct, but later 
Tryon, Verrill, Dall, and others had come to regard A. alveus as only 
a variety of A. testudinaUs. Mr. Henry Jackson, Jr., (3) in a com- 
munication to the "Nautilus," published the results of a very careful 
study of the radula of the two forms, and shows marked differences 
between them. So far as I have observed testudinaUs occurs in pools 
at low tide exposed to dashing waves. I have never seen a specimen of 
this species on eel grass ; alveus on the contrary lives on eel grass in 
quiet water and in certain places hundreds may be collected in a short 
time. It was naturally believed by some observers that the long 
narrow form of alveus had l)ecome so by adaptation to its narrow rest- 
ing place; if so, it is a good example of a species in the process of 
establishing itself. Whatever may be the case the specific characters 
are now so firmly fixed that I have never seen a specimen, young or 
