Bactrites are a genus of straight shelled (orthoconic) Cephalopod. The shells tend to be long and narrow in appearance. Unique to Bactrites, the siphunicle runs close to the outside of the shell. The order of Bactritida has a temporal rage of 407 to 231 million years of age, therefore it fits in my time period. However, I have yet to find any evidence of Bactritida being found in Pennsylvania.
What is a siphunicle?
In Cephalopods, the siphunicle is a strand of tissue that runs the length of the shell. With Mooreoceras, the siphunicle is located on the inside of the shell. In Bactrites, it runs nearly along the outside of the shell. The siphunicle allows the creature to control it’s buoyancy and thus its position and elevation in the water.