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 (Phragmoceras, Gomphoceras, etc.) ; 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 (Actinoceras), arid 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 



