28 E. &. Morse—Classification of eases 
end downward. In Calyptrasn they are in a fixed position, 
pecreting a ine valve, upon which they rest. It would be 
interesting to know for a certainty which part first becomes 
attached in Vermetus and allied forms; their, first point of 
attachment must take place at the mouth of the tube or a aper- 
ture, which is really anterior and ventral. The Cephalopods 
are free. 
Thus we have the various regions of attachment, changing 
and wel in the diencsion indicated by the arrow A, in 
cend in structure toward the anterior end, so we find the princi- 
pal organ of locomotion, i.e., the foot, is first developed from 
the ventral region, and in like manner tending toward the ante- 
rior end, as we ascend in the scale, until, in Cephalopoda, = 
ed divisions of the foot surround the head, and poin 
irectly forward. 
aving personally communicated the substance of this paper 
to Professor James D. Dana, he has, in a letter to me, indicated 
certain gradient ieleaon: among the Lamellibranchs, Gastero- 
pods, and Cephalopods, as manifested in the special characteris- 
tics of the — or anterior part of the body, so clearly illus- 
trating the principle of cephalization that I now take the liberty 
of presenting them. In the Lamellibranchs the foot is a simple 
muscular organ developed from the ventral surface and protrud- 
ing anteriorly. It is simply an organ of locomotion, in the 
lower forms not even performing this faneliote The oral open- 
ing = simple slit, without the power of seizing or triturating 
its 
