94 
NAUTILOIDEA. 
Sjy. Char. The following is ]VI‘Coy’s description of the type 
specimen : — Discoid, compressed, whorls quadrangular ; sides flat, 
smooth ; back flat, smooth, three fourths the width of the sides ; 
inner whorls exposed, about five in number ; septa simple, about 
half an inch apart. 
“ This fine species often reaches to ten inches in diameter ; it is 
distinguished from the N. complanatus^ Sow., by its very broad, flat 
back and simple septa ; from the N. discus, Sow., by the broad, flat 
back and the greater spaces between the chambers ; the N. quad- 
ratus, Elem., is distinguished by its striated surface. Diameter, 
four inches ; width of last whorl, one and a half inches ; breadth of 
back, one inch three lines.” (M^Coy.) 
The transverse section is nearly quadrate in the young shell, but 
becomes elongated in the adult, on account of the more rapid growth 
of the shell. The umbilicus has a small central perforation. 
The septa are not very approximate, they number from twenty- 
one to twenty-five. The sutures make a rather deep backward 
curve on the sides of the shell, and a moderately deep sinus upon 
the periphery. 
The body-chamber is rather large, and occupies nearly half of the 
last whorl. 
The surface of the test is ornamented only with fine strice of 
growth corresponding in direction with the outline of the aperture. 
RemarJcs. This species was at first called liexagonus by de Kon- 
inck \ but as that name had already been used by J. Sowerby (1837) 
for a Jurassic species, it had to be abandoned, and in the meantime 
M‘Coy ^ had described it under the name of planotergatus, which it 
now bears. 
D. planotergatus differs from D. Leveilleanus in its more rapid 
rate of increase and more angular whorls, and also in the absence 
of longitudinal ridges upon the test, which are a conspicuous feature 
in the ornamentation of the latter. 
A specimen in the Collection, agreeing perfectly in its septal 
characters, and also in the position of the siphuncle with D. plano- 
tergatus, nevertheless differs from it in being a more compressed, 
thinner shell, and having much steeper sides to the umbilicus. 
This specimen (No. C. 1544) is a section cut through the centre of 
the whorls, and is from the Carboniferous Limestone of Derbyshire. 
It might perhaps be regarded as a compressed variety of planoter- 
gatus. ' 
^ Descrip, des Anim. Foss, du Terr. Garb, de la Belgique, pi. xxv. f. 1. 
^ Synop. Oarb. Foss. Ireland, p. 18, pi. ii. f . 2. 
