7 6 4 



THE POPULAR SCIENCE MONTHLY. 



Fig. 6. Spirttla. 



ress toward that position. The history of their past begins with the 

 recognition of the pearly nautilus (Fig. 3) as a being which, as a four- 

 gilled cuttle-fish possessing an external many-chambered shell, stands 

 alone in the world of life. It is the tribes of two-gilled cuttle-fishes 



which people our ocean to-day, and which -exhibit 

 ail the gradations of form and size, from the 

 minute Spirilla (Fig. 6) to the great Architeuthis 

 of the American coasts. The history of the cut- 

 tle-fishes in time begins in the far-back epoch 

 represented by the Lower Silurian rocks of the 

 geologist. There are entombed the first fossil 

 cuttle-fishes, represented by their chambered 

 shells. The genus Orthoceras, represented by 

 shells of straight form, is thus among the oldest 

 members of the cuttle-fish race. The Nautilus genus itself begins in 

 the Upper Silurian rocks ; we may trace the well-known shells upward 

 to the Carboniferous strata, where they are best developed ; and we 

 follow the genus onward in time, as it decreases in numbers, until we 

 arrive at the existing order of things, in which the solitary nautilus 

 remains, as we have seen, to represent in itself the fullness of cephalo- 

 pod life in the oceans of the past. The older or Paleozoic rocks re- 

 veal a literal wealth of these chambered shells, and therefore of the 

 existence of the four-gilled cuttle-fishes as the founders of the race. 

 When we ascend to the Mesozoic rocks (ranging from the Trias to 

 the Chalk), we meet with new types of the chambered shells well-nigh 

 unknown in the Palaeozoic period. In the Mesozoic rocks appears the 

 fullness of Ammonite life. Here we find shells named after the horns 

 of the Egyptian god, Jupiter Ammon ; these, instead of being toler- 

 ably plain, like the Nautilidw, exhibit beautifully sculptured outlines, 

 and folded septa, or partitions, between the chambers of the shell. 

 The shells allied to Nautilus and occurring in the Palaeozoic formations 

 differ from Nautilus chiefly in their varying degrees of curvature or 

 straightness. Lituites is a curved form allied to Nautilus / while 

 Orthoceras and Gomphoceras are groups representing the straight- 

 ened forms. But in the Silurian period more complex forms appear, 

 with elaborate and folded septa. These are the early Ammonites, 

 such as Goniatitcs and Haetrites. In the Secondary rocks we find 

 the still more complex true Ammonites themselves. Here the lobes 

 and saddles of the shells, as the edges of the septa are named, are of 

 the most elaborate patterns, while the shapes of shell are of the most 

 varied character (Eaculites, Turrilites, Ammonites, etc., Fig. 5). 



There is thus an advance and progression exhibited in the develop- 

 ment of the four-gilled races which accords perfectly with the theory 

 of evolution and descent. The seas of the Trias, Oolite, and Chalk 

 periods must have literally swarmed with these striking forms of 

 cephalopod life ; but as the close of the Chalk period dawned, and as 



