CEPHALOPODA. 



10^ 



No. 242. [545, Cast]. Baculites anceps. Lam. 



The Ammonitidae, whose forms have been gradually more and 

 more unrolled as we ascend in the geological series, terminate in the 

 Upper Chalk with this perfectly straight form. The shell of this fos- 

 sil seems to have been very thin, and usually, as in the present speci- 

 men, the cast of the chambers alone remains. The original, from 

 the Upper Chalk of France, is in the Ward Collection of the Uni- 

 versity of Rochester. Size, 19 x 3. 



No. 243. Baculites compressus, Lay. 



This species is larger and less slender than the above, and the specimens 

 show a portion of the outer shell. From the Cretaceous, Black Hills, Dakota. 



FAMILY NAUTILIDiE. 



This family includes those chambered cephalopod shells having 

 simple, concave septa, smooth sutures, and an internal, simple, 

 nearly central siphuncle. With the exception of Nautilus and 

 Aturia^ it is almost entirely confined to the Palaeozoic rocks. 



The first true Nautilus with regular spiral appeared in the 

 Upper Silurian ; and the Nautiloid form has survived to the 

 present day. The family culminated in the Carboniferous Age. 

 The Nautilus is the only representative in the existing seas of 

 that vast assemblage of chambered siphonated shells which 

 swarmed in the ancient oceans. There are six living species of 

 Nautilus, pomjyilius being the common one, and nearly 300 

 fossil species. 



The horny beaks of the family are frequently found fossil ; 

 the upper beaks are called Rhyncholites and the lower ones Con- 

 cliorhyncKxLS. 



No. 244. [570, Cast]. Nautilus elegans, Sow. 



This very characteristic species of the Lower 

 Chalk is from Havre, France. 



Size, 10 X 9. 



