22 



Guide to the Invertebrata. 



VII 



Wall 



Ortboceras Althougli the fossil representatives of this section of the 

 ^^^?^^ Cephalopoda (iN'autiloidea) are very numerous and varied in 

 form, they always have simple suture-lines. The simplest form 

 ( OrtJioceras) is like a Nautilus unrolled ; it has a straight, or 

 only slightly bent, more or less conical tube, ornamented some- 

 times with longitudinal, sometimes with 

 transverse ridges, or merely with trans- 

 verse striee, and divided into chambers by 

 transverse partitions. The last chamber is 

 relatively large and contained the animal ; 

 a tube (the siphuncle) extended from the 

 hinder portion of the animal through 

 each partition to the first chamber of the 

 shell, just as in the recent Nautilus. 

 These animals probably crawled along 

 the sea-bottom like the living Nautilus, 

 carrying their shells in a nearly vertical 

 position. This genus, some species of 

 which were several feet in length, ranges 

 from the Cambrian to the Trias, being 

 especially abundant in the Silurian rocks, 

 a very large number having been described 

 from the Silurian rocks of Bohemia. 

 Numerous examples of these straight 

 septate shells may be seen in Wall- 

 case 8, among them being several ex- 

 cellent examples of an Orthoceras from 

 China {Orthoceras Chinense), there known 

 as "Pagoda stones" from the popular 

 belief that they are formed underground 

 where the shadow of a pagoda has fallen 

 upon the surface. Some of these are 



turn Boll, a example sectioned and polished, and show the 



Avith some ot the test . , . , ' <. . ,, 



adherent; i, section with central tube or siphuncle perforating all 



siphuncle ; c portion of the chambers. A large example of the 



test enlarged. XJ. Silu- . . _ ^ ^ 



rian : Gothland. same species may be seen fixed up at 



the end of the same wall-case. 

 On the wall between this wall- case and the doorway is a fine 

 polished slab from Bohemia, exhibiting numerous examples of 

 these straight-shelled forms. 



Fig. 42. — Orthoceras orna- 



