134 



MOLLUSCA. 



Orthoceratidse. 



These Cephalopoda were the princes of Molluscs in the Palaeozoic 

 times. They resemble in general uncoiled Nautili, being conical shells 

 divided by simple septa, concave next the outlet. The siphuncle is usually 

 eentral, but sometimes marginal, and always complicated. The shell is 

 sometimes curved, but never compactly coiled as in the Nautili ; and the 

 animal was probably unable to withdraw itself into the shell — the body- 

 chamber being relatively smaller than that of the Nautili. The shell, 

 however, was essentially external. These Tetrabranchs were the most 

 abundant and wide-spread molluscs of the old rocks, and attained a larger 

 size than any other fossil shell. They were probably the chief agents 

 in performing 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. 



No. 584. Orthoceras Titan, Hall. 



It was with this entirely straight, staff-like 

 form that the family of the Orthoceratidce made 

 its first appearance on our globe. In this feature 

 they present a picture quite the converse of the 

 Ammonitidce, which began with the tightly coiled 

 forms, and closed with the straight Baculites. Some 

 individuals of this genus grew to an immense 

 size, and their movement in the water would have 

 been extremely slow and cumbrous, but for the 

 many air-chambers which buoyed and floated their 

 shell. This gigantic specimen shows the last 

 chamber, the greatest diameter of which is over 

 eight inches ; and the length of the entire original 

 must have been about nineteen feet ! From the 

 Trenton limestone (Lower Silurian), Lowville, 

 New York. 



Size of slab, 9 ft. x 1 ft. 1 in. Price, $10.00. 



N®. 585. Orthoceras amplicameratum, Hall. 



On slab. This handsome specimen from the Trenton limestone (Lower 

 Silurian) of Middleville, N. Y., shows the entire length of the last chambers, with 

 faint traces of the mouth. The shell being gone, the chambers of the posterior 

 portion are visible. The original is now in the Ward Museum, University of 

 Rochester. Size, 9x4. Price, $1.00. 



