8 NATURAL <SChEN GE. JAN., 
Another feature of the Neritide is the presence of a thick shelly 
deposit (the callus) just without the mouth,.on the surface of the last 
whorl. This deposit is thickest at the margin next the mouth, 
and generally at this point forms a ridge or even a thin shelf, or 
septum, which stretches partly across the mouth, reducing it in 
size. The full edge of this shelf is often serrate, sometimes coarsely 
so. As the animal grows and adds to its shell, still winding round 
in a spiral direction, these various deposits advance with it by the 
simple process of the addition of fresh material on their outer sides 
(using the word “outer” in its relation to the mouth and the 
direction of growth), and the removal of a corresponding amount on 
their inner sides. 
Having prefaced so much, it is easy to understand the interest of 
what took place in the most eccentric Eocene member of the family, 
Velates conoideus, Lamk., as described by Mr. Woodward. In this 
species the growth of the young shell was perfectly normal, but when 
it had completed about 44 whorls a remarkable change ensued ; it 
ceased to grow spirally, and increased by the addition of fresh layers 
of shell over the whole surface of the callus as well as round the 
margins of its mouth, and gained the additional internal space the 
animal required by the removal of a corresponding amount of shell on 
the inner side of the callus. It thus changed both its direction and axis 
of growth—and like the Irishman, it raised its roof by digging out the 
floor of its tenement. In a full-grown specimen, therefore, nearly half 
the shell and the internal septum were carved out of former callus, the 
layers of which can be seen in sections cutting across the walls and 
running round on the inner surface. A still more remarkable feature 
is presented in the disposition of the component plates which, in the 
callus, are so arranged as to provide, in anticipation, the strongest 
possible structure for the muscle-carrying septum, ultimately to be 
sculptured out from it. The presence of silica in the outer layer of 
the shell is also noteworthy, and it is interesting in this connection to 
remark that one of the earliest describers of this fossil gastropod, and 
the author after whom it is sometimes called—C. C. Schmidel— 
observed, in 1780, that this outer layer did not appear to consist 
entirely of lime. 
THE CLASSIFICATION OF BIVALVED SHELLS. 
AFTER the embryologist has made a certain advance in the study 
of development, and the comparative anatomist has progressed in 
the determination of homologous parts, the services of the systematic 
zoologist are required to express the results in orderly sequence. 
Classifications are thus merely temporary expedients—generalised 
retrospects, so to speak—and every advance demands some recon- 
sideration. At the present time Professor Carl Grobben (of Vienna) 
is of opinion that some reform is necessary in the classification of the 
