CEPHALOPODA 85 



ture also exists and is very regular in the siphnncle of the 

 Devonian Orthoceras trigonale, and in the shells referred to 

 Oyroceras by d'Orbigny (fig. 20, s) ; also in Actinoceras, a sub- 

 genus of Orthoceras, discovered by Dr. Bigsby, and described by 

 Stokes (Geol. Trans., vol. i., 1825). The plication of this in- 

 terior structure takes place in segments corresponding to the 

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

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

 the lining membrane of the air-chambers* The vascularity 

 of the latter is well shown in the impression of septa on the 

 fine mudstones of the Ludlow rock, often mistaken for Spon- 

 garia, 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 bisijjhonahim (Tretoceras, Salter) the body- 

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

 ing a second siphuncle. 



The genus Bactrites of Sandberger also resembles an Ortho- 

 ceras with single-lobed sutures. 



Ammonitidcc. — In the second division or family of cham- 

 bered shells — those with lobed sutures and a marginal si- 

 phuncle — we find a similar series of forms, straight, spiral, 

 and discoidal, but more varied and more highly ornamented. 



One large genus (Goniatites, fig. 20, 10) is found in the 

 Devonian, carboniferous, and triassic strata, and permanently 

 resembles the youngest form of the Ammonites, having the su- 

 tures lobed but not foliated. They seldom exceed 6 inches in 

 diameter and are usually very much smaller. The whirls are 



* In the carboniferous species of Actinoceras (e.g., A. giganteum), these for- 

 amina form a cross on the ventral side of the siphuncle. 



