96 



MOLLUSCA. 



and Dr. Paul Fischer would raise the family to the rank of ai> 

 order. Excepting the Goniatites, the family is peculiar to, and 

 co-extensive with, the Secondary strata. AmTnonites proper in-^ 

 habited 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. Accord- 

 ing to d'Orbigny, the compressed specimens are males, and the 

 inflated, females. Supposed opercula {Aptychi) are frequently 

 found associated with these shells. The shell of the ATnmonites 

 is generally thinner and more delicate than that of the Naxitihvs ; 

 the partitions are consequently moi'e 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. ATnmonites are rare in tlie 

 Jurassic of America, but abound in Europe, and they have been 

 seen in Oolitic deposits on the Himalayas, 16,200 feet above the 

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

 living Nautilus. 



No. 183. G-oniatites Ixion, Hall. 



The Goniatites were the earliest of the family Ammoni- 

 tidae. This specimen closely resembles O. rotatorius of 

 DeKoninck, and is a very characteristic fossil of the Goni- 

 atite Limestone at Rockford, Indiana. 



No. 183. G-oniatites Oweni, Hall. 



The most prominent feature separating this species from the preceding is 



the presence of an umbilicus. 



Goniatite Limestone, Rockford, Indiana. 



No. 184. Ceratites nodosus, De Hann. 



This genus is distinguished by having 

 the lobes of the sutures serrated, while the 

 intervening "saddles" are simple. This 

 specimen is from the Muschelkalk (Trias),^ 

 at Niederbronn, Alsace. 



