CEPHALOPODA. 



117 



Ammonitidse. 



This is the most remarkable family of Secondary Molluscs. Some 

 twenty or more genera and nearly 1000 species have been identified. 

 They range from the Devonian to the Chalk inclusive, becom- 

 ing totally extinct at the close of the Reptilian Period. The family 

 includes the following principal genera in the order of their appearance : 

 Goniatites, Ammonites proper, Ancyloceras and Helicoceras, Crioceras, 

 Toxoceras, Scaphites, Hamites, Baculites and Ptychoceras, and Turri- 

 lites. The type began in the discoidal Goniatites, culminated in the 

 compactly coiled and elaborate Ammonites, and expired in the half un- 

 coiled forms as Scaphites, the spiral Turrilites, and the straight Bacu- 

 lites. 



This division of chambered shells is distinguished from the succeed- 

 ing family by more varied and more highly ornamented forms, with 

 crumpled septa, lobed sutures, and marginal siphuncle. Excepting 

 the Goniatites, the family is peculiar to and co-extensive with, the 

 secondary strata. Ammonites proper inhabited involute shells having 

 undulating septa, lobed and foliated sutures, a dorsal siphuncle 

 (ventral as regards the animal), and a small nucleus, the whorl 

 being compact from the first. The pattern is constant in each species. 

 The shells are most beautiful when of middle growth, the ornamental 

 characters being less developed in the young and lost in the adult. 

 According to D'Orbigny, the compressed specimens are males, and the 

 inflated, females. Their fossil beaks (Rhyncholites) are claimed to have 

 been lately discovered. The shell of the Ammonite is generally thinner 

 and more delicate than that of the Nautilus ; the partitions are conse- 

 quently more complicated, and the ribs are adorned and strengthened 

 with spines, tubercles and bosses. With few exceptions, those having 

 the back keeled with a furrow on each side mark the Lias period ; while 

 those with sharp, square or round backs are Oolitic. Ammonites are 

 comparatively rare in America, but abound in Europe, and they have 

 been seen in Oolitic deposits on the Himmalayas, 16,200 feet above the 

 sea. The Jurassic Ammonites were of far higher grade than the 

 living Nautilus. 



No. 467. Ammonites armatus, Sowerby. 



This species is typical of Von Buch's Group 

 "Armati." From the Lower Lias, Charmouth, 

 England, and now in the private Geological 

 Cabinet of Mr. Ward, Rochester. 



Price, $0.50. 



