Silurian Age of Mollusks. 75 



however, the shell is not all in one solid piece, but in eight transverse 

 segments. Beginning at the front end, each segment is grown to the 

 pallium or mantle by its forward margin, while its Binder margin laps 

 over the segment or plate behind it. The G-asteropod had a toothed 

 tongue and a heart. The heart may have only one chamber, a ventricle, 

 which is the case with the Natica, one of the families represented in the 

 upper Potsdam, or it may have two cavities, an auricle and a ventricle, 

 sometimes near together, but often in different parts of the circulatory 

 system, of which they constitute merely swellings or enlargements of 

 the vessels. 



The Turbinidse, a family related to the Natica, is represented by the 

 Trocus genus in the Potsdam. The G-asteropods also belong to Hseckel's 

 Cochlides. 



Next, and highest of the Mollusks, are the Cephalopods. To this 

 class belong the modern Sepia or cuttlefishes, the Octopus or Devil fish, 

 the Nautilus, and Loligo or squid. The Orthoceratjdae, the Belemnitidse, 

 and the Ammonitidse are fossil and extinct. The Orthoceratidse began 

 in the Potsdam period, and disappeared in the Triassic. Some of the 

 Nautilus family are also extinct, some beginning and ending in the Silu- 

 rian age. The Orthoceras is like a straight horn, hence the name. It 

 was a straight cjdindrical or rather tapering shell, divided internally by 

 cross partitions into a series of segments. A tube or siphuncle passed 

 through all these partitions, from end to end of the animal. This 

 siphuncle is, in some species, in the center, in others, dorsal or nearer 

 the back, in others, ventral or in the lower part. The animal was com- 

 posed of metamera, or sections, which were connected with each other 

 through the siphuncle. 



The modern cephalopods have eight or ten arms or tentacles arranged 

 around the mouth, they have two or four plume-like gills within the 

 mantle, they have an orifice in front under the mouth called the funnel 

 ( inf undibulum), through which the effete matters are voided in their 

 water respiration. They are all naked except some species of the Nau- 

 tilus, but some have a calcareous or flexible, horny skeleton of simple 

 structure, inside. They have eyes and nervous throat ganglia with nerve 

 filaments running to the organs. Their circulation is kept up by the 

 action of two or three contractile chambers connected with, or part of, 

 the blood vessels hearts. 



It is impossible to determine just how much of this internal machinery 

 was possessed by the ancient ortheceras and his relations. His circula- 

 tory and nervous s} 7 stems, at least, were much simpler. His relatives 

 of the family of the Orthoceratidse differed from him chiefly in their 

 external form, as they were curved like a horn in the Cyrtoceras, curved 

 into a disc or flat scroll in the Gyroceras. These variations of form are, 



