92 
NATJTILOIDEA. 
1860. Discites mutahilis, Griffitli, Journ. Geol. Soc. Dublin, vol. ix. 
p. 77. 
1878. Nautilus mutahilis, cle Koninck, Faune de Calcaire Carbonifere 
de la Belgique (Annales du ^lus. Roy. d’llist. Nat. tom. ii.), pt. i. 
p. 121, pi. XXV. fF. 2 a, 2h, 2 c. 
Sj). Char. “ Discoid, very much compressed ; whorls five or six \ 
entirely exposed, quadrangular, aperture sagittate ; back very con- 
cave, less than one fourth the width of the side, umbilical slope as 
broad as the back, steep, concave, with an angular margin ; two 
outer whorls in adult specimens perfectly smooth, inner ones marked 
with strong, equal, spiral sulci. 
“ The change of character of the surface from the inner to the 
outer whorls is even greater than in the Nautilus {Discites) discors, 
M‘Coy, the inner volutions being spirally striated as in that species, 
but the outer turns being perfectly smooth ; it differs from that 
species, besides the smoothness of the outer whorls, in the much 
more compressed form, long narrow mouth, and the narrow and 
very concave hack ; from the N. (Discites) trochlea, M‘Coy, it differs 
in the striation of the inner whorls . . . (M^Coy.) 
Remarks. In well-preserved specimens it is seen that the surface 
of the test in the adult is not perfectly smooth, as described by 
M‘Coy, but that there are fine transverse lines which form a sig- 
moid curve upon the sides of the shell, and a backwardly directed 
sinus upon the periphery. The siphuncle is a little above the centre, 
and retains that position throughout the whole extent of the shell. 
The outline of the aperture, which is of a sigmoid form with a 
rather deep peripheral sinus, is preserved in one of the specimens in 
the Collection ^ In approaching the aperture the body-chamber 
disengages itself from the coiled portion and continues for a greater 
or less distance in a larger curvature. In one specimen (No. 43867) 
the free portion is at a maximum distance of 8 lines from the coiled 
part. 
There can be very little doubt that the Ellijpsolites compressus, 
Sowerby, is identical with the present species, and therefore M‘Coy’s 
name must give place to the former. It is true that the specimens 
upon which Sowerby founded his species are in the worst possible 
state of preservation, the larger one being not only distorted, but 
badly eroded as well ; yet the general characters — number and shape 
^ M‘Coy overestimated the number of whorls, owing probably to his speci- 
men having the inner ones covered by the matrix. There are not more than 
3|, or 4 at the most. 
^ No. 1685 in the Register. 
