510 MOLLUSCA SUB-KINGDOM VI 
Dimerites (Fig. 1084), producing a T-shaped opening; or, as in Hercoceras 
(Fig. 1071), it may occur principally from the dorsum and venter, resulting in 
a transverse aperture. The dorsal side of the aperture is, as a rule, occupied 
by a crest, known as the dorsal crest (Figs. 1066, 1072, 1089). The position 
of the hyponome is indicated by the large single opening and sinus at the 
termination of the longer median slit of the aperture in shells with contracted 
openings that obviously had this organ (Phragimoceras, Gomphoceras, ete.) ; but 
in others like Hercoceras, which have no ventral sinus in the aperture, the 
hyponome was probably absent or non-functional. The sinus in the lines of 
growth, however (Fig. 1071), show that this organ was present in the pre- 
ceding stages of development before the contracted apertures were formed. 
Dr. Pompeckj states that contracted apertures occur only in the senile 
stages of growth, and small shells having this peculiarity must be regarded as 
dwarfs. This is certainly true of many species, and is probably also the case 
with Hercoceras and the like. T-shaped apertures often show several accessory 
sinuses and crests (Fig. 1085), which probably indicate the number of their 
protrusible arms or tentacles. Most curved forms have the ventral sinus on 
the arched external side (exogastric shells), but some have it on the concave 
internal side, as in Phragmoceras, and these are called endogastric shells. The 
interior wall of the living chamber, and volutions in recent and _ fossil 
Nautiloids (Fig. 1073), are normally marked with fine transverse and longi- 
tudinal lines. In the recent Nautilus a black superficial layer, composed in 
part of organic matter, is deposited by the hood immediately in front of the 
aperture on the dorsum. 
The internal partitions or septa, which divide the volutions into chambers, 
_vary exceedingly in number among different species and also at different ages 
of the same individual; but they are tolerably constant. as a rule, within the 
limits of one and the same species, if specimens of the same age are compared. 
They follow one another in regular succession, but as observed by Hyatt, the 
intervals are relatively greater in the young, more constant in the adult, and 
then markedly decrease in the oldest stage of development. Each septal 
chamber (camera of Hyatt) was part of the living chamber until it was cut off 
by a septum and left empty as the animal moved forward. Perfectly pre- 
served shells may have the living chamber alone filled up with stony matrix, 
since the sediment could only pass into the preceding chambers through the 
siphuncle, or as a result of injury to the walls of the camerae. Nevertheless,» 
these last are seldom entirely empty, their interiors being frequently lined 
with crystals of infiltrated calcite, quartz, celestine, baryte, pyrite, or with 
organic excretions. Double septa occur in some forms (detinoceras), and in 
others the camerae are sometimes secondarily partitioned off by intermediate 
walls or pseudo-septa, which may either run parallel with the septa proper, or 
at an angle with them, and are composed of two readily separable calcareous 
lamellae. The origin of these pseudo-septa has been attributed to the 
calcification of regularly arched membranes at the posterior end of the body. 
The line of junction between the septa and inner wall of the shell is called 
the suture. This is invisible externally, except when the shell-substance has 
been broken or worn, or dissolved away, and it is seen most clearly on natural 
casts. The sutures of Nautiloid shells follow, as a rule, simple, straight, or 
slightly undulating lines. These undulations, when convex toward the apex, 
are termed lobes, and the reversed or orad curves are the saddles. ‘They are 
ee 
