487 
the dorsal side, the centre of the dorsum again begins to round 
out, but traces of the primitive dorsal furrow remain in the de- 
pressions on either side of the central, gibbous dorsal face formed 
by this outgrowth, as in Fig. 24. 
This gibbous face is immediately suppressed when the whorls 
come into contact, and its transient appearance can only be 
accounted for as due to the genetic tendency of the paranepionic 
whorl to resume the gibbous metanepionic form of dorsum as soon 
as the pressure resulting from the abrupt curve is slightly relieved.* 
TROCHOLITES AMMONIUS, Hall. 
This species, of which the collection of the Museum of Compara- 
tive Zodlogy possesses a very large number, collected by Mr. C. D. 
Walcott, has a very peculiar, rough, fretted surface, and only very few 
specimens show longitudinal lines such as are described and figured 
in Z: planorbiformis by Conrad. ‘This surface is due to the minute 
crenulations or waves in the outlines of the projecting edges of the 
laminz of growth. When these are wide enough apart one can 
distinguish crenulated transverse lines; when too close they inter- 
fere and the regularity and continuity of the lines are broken into 
a multitude of more or less discontinuous, short lines. Sometimes 
a network of lines is formed by the regularitv of the intersection 
of the crests of the crenulations in successive laminz. This cuticu- 
lar ornamentation is so easily destroyed that it is often present only 
on parts of the same specimen. 
Longitudinal lines may be seen through it, but, as stated by Hall 
and observed by the writer, these are rarely present in the New 
York specimens. They do, however, sometimes exist all over the 
abdomen and sides, and are well defined in specimens in which the 
cuticular corrugations are absent. 
The lines of growth are extremely crowded, and what are called 
the cost occur at wider intervals and more irregularly. ‘They are 
probably the traces of former apertures. These are more promi- 
nent in some specimens than in others, but never seem to have the 
aspect of true fold-like costations. 
The lines of growth form deep, broad sinuses on the venter ; 
rise into lateral crests on the sides, sinking towards the lines of 
involution, and forming a sinus in the contact furrow. ‘These are 
* This opinion would be more convincing, if it were not for the fact, that in Cranoceras 
similar transformations occur in an adult cyrtoceran form of the Devonian. 
