102 PALEONTOLOGY. 



interior structure takes place in segments corresponding to 

 the septa, meeting in the centres of the siphuncular beads, 

 and leaving spaces or foramina for the passage of blood-vessels 

 to the lining membrane of the air-chambers. In the carboni- 

 ferous Actinoceras giganteitm, these foramina form a cross on 

 the ventral side of the siphuncle. The vascularity of the 

 lining membrane is well shewn in the impression of septa on 

 the fine mudstones of the Ludlow rock, often mistaken for 

 Spongaria, which they somewhat resemble. 



Towards the conclusion of its growth the air-chambers of 

 the Orthoceras frequently become shallower, and the siphuncle 

 diminishes in size. These indications of changed or diminished 

 energies are accompanied by a diminution or disappearance 

 of the internal radiated structure in the last part of the 

 siphuncle. 



In Orthoceras bisiphonatum (Tretoceras, Salter) the body- 

 chamber is prolonged in the form of a marginal lobe, simulat- 

 ing a second siphuncle. The genus Bactritites of Sandberger 

 resembles an Orthoceras with single-lobed sutures. 



Ammonitidce. — In the division or family of chambered 

 shells, with lobed sutures and a marginal siphuncle, we find 

 a similar series of forms, straight, spiral, and discoidal, but 

 more varied and more highly ornamented. 



One large genus (Goniatites, fig. 30, 10) is found in the 

 Devonian, carboniferous, and triassic strata, and permanently 

 resembles the youngest form of the Ammonites, having the 

 sutures lobed but not foliated. They seldom exceed 6 inches 

 in diameter, and are usually very much smaller. The whorls 

 are most frequently concealed to some extent, and often marked 

 by cross furrows or " periodic mouths." 



The Ceratites are distinguished by having the lobes of the 

 sutures serrated, while the intervening "saddles" (or curves 

 directed towards the aperture) are simple. They are found 

 in the trias of Europe, Thibet, and South America ; and again 



