Bactrites are a genus of straight-shelled (orthoconic) cephalopod. The shells tend to be long and narrow in appearance. Unique to Bactrites, the siphuncle runs close to the outside shell surface. The order of Bactritida has a temporal range of 407 to 231 million years of age; therefore, it fits in my period. However, I have not found evidence of Bactritida being located in Pennsylvania.

What is a siphuncle?

In Cephalopods, the siphuncle is a strand of tissue that runs the length of the shell. With Mooreoceras, the siphuncle is located on the inside of the shell. In Bactrites, it runs nearly along the outside of the shell. The siphuncle allows the creature to control its buoyancy and thus its position and elevation in the water.

Siphuncle in a Cephalopod. Bactrites has it located at the outside shell edge.
The siphuncle is shown in yellow on a coiled Cephalopod.
Original Picture © BIODIDAC 2004.


Ctenobactrites is a genus of cephalopod under the order of Bactritida, an order rarely found in Western Pennsylvanian rocks. The genus Ctenobactrites differs from Bactrites by having coarse transverse lirae (Mapes 1979). The bands increased in height as the specimen grew longer. The shell is orthoconic. Reports of the genus are rare in Pennsylvanian rocks of North America.

Several authors have searched for Meek’s physical holotype C. isogramma and came up empty-handed. Mapes (1979) erected Ctenobactrites for a specimen of Bactrites with a sole coarse-banded patch of shell remaining. The coarse ornament can resemble growth lines on a bivalve. The fragments also superficially resemble portions of a gastropod shell. But, they fail the test upon closer examination.

Subgenera of Ctenobactrites

Shimansky (1954) erected subgenera, yet Mapes (1979) observed that some of the characteristics, such as rounded or flattened lirae, exist in particular specimens; thus, subgenera naming is not to be used.


More Reading and Research on Bactrites