108 MOLLUSC A. 



No. 345. [574, CastJ. Nautilus pseudo-elegans, d'Orb. 



This tiue specimeu is an inner cast, and 

 shows the septa of the shell. From the Chalk 

 of Rouen, France, and now in the Ward Col- 

 lection, University of Rochester. 



Size. 6 X 5. 



No. 246. Nautilus semistriatus, d'Orb. 



This large specimen is from the Middle Lias, Charmouth, England. 



No. 247. [595, Cast]. Nautilus Maximus, Conrad. 



This species is more loosely coiled than any of the preceding. The umbili- 

 cus is wide and deep, and the chamber of habitation is verj' large. From the 

 Hamilton Group (Devonian), of Madison Co., N. Y. Diameter, 14. 



No. 248. [580, Cast]. Lituites undatus, Conrad. 



This rare and remarkable shell is a Palfeozoic ally of 

 the jyautilus, from which genus it differs mainly by the 

 complete exposure of all its whorls, and the straight- 

 uess of the last chamber. It appeared in the earliest 

 Silurian. From the Black River Limestone (Lower 

 Silurian), Middleville, N. Y. 



FAMILY ORTHOCERATIDiE. 



Tliese tetrabranch ceiilialopods were the ]3riiices of mollnsks 

 in Palaeozoic times, being more abundant and attaining a larger 

 size than any other fossil shell. They resemble in general mi- 

 coiled JVautilidw, being conical shells divided by simple septa, 

 concave next the outlet. The sij^hnncle is usually central, but 

 sometimes marginal, and always complicated. The shell is some- 

 times curved, but never completely coiled as in the JS^atitilus ^ 

 and the animal was probably unable to withdraw itself into the 

 shell — the body chamber being relatively smaller than that of 

 the Nautilus. The shell, however, was essentially external. 

 They appear in the Cambrian, culminate in the Silurian and dis- 

 appear in the Triassic. 1200 species have been named. Some 

 authors include them under Nautilid?e. 



These tetrabranchs were ]3robably the chief agents in perform- 

 ing the duties allotted to the marine carnivora. The New York 

 specimens are most numerous in the Trenton Limestone. Those 

 with a large siphuncle are wanting in England, France, Ireland 

 and Bohemia, but abound in America, Russia and Sweden. 



