2G0 PROCEEDINGS OF THE ACADEMY OF 



the septa are cup-shaped, as in Orthoceras, but, unlike that genus, 

 the}^ are not perforated by a small siphonal opening. Instead, 

 the middle of each septum is prolonged into a tapering tube with 

 an elliptical cross section, tbe apex of which tube enters the base 

 and nearly, or completely, fills the interior of its predecessor. 

 Extreme apex unknown. In the space immediately succeeding 

 tbe last septum, there is a saddle-shaped, continuous muscular (?) 

 scar, which rises on the sides corresponding with the broad part 

 of the funnel-like siphonal tubes, and is deeply depressed on the 

 sides corresponding with the ends of the ellipse. Beyond this 

 stage, the shell continues as a nearly straight tube, increasing 

 very slowly in diameter and, at irregular intervals, is hermetically 

 closed by convex septa, having their convex face reversed ; that 

 is to say, placed towards the broader, or newer portion of the 

 tube. The structure is microscopically prismatic, the prisms being 

 placed transversely as in Inoceramus. Both the walls and the 

 septa are made up of numerous layers of shell substance, no struc- 

 tural difference existing between the several parts. 



P. tibialis grows in masses of tubes bearing a strong superficial 

 resemblance to Teredo, which resemblance misled Dr. Morton m 

 his generic reference. But, even apart from its internal structure, 

 the analogy fails, since it never occurs perforating wood or other 

 hard substances. The spaces between the shells are filled only 

 with sand. This free mode of growth, and my not being ac- 

 quainted at the time with the peculiar characters of the apical 

 portion, induced me to refer my genus to the Vermetidse on ac- 

 count of the septa in the larger portions of the tube; a character 

 not unlike that of Vermetus and Caecum. Latterly, through the 

 kindness of my friend, Dr. Joseph Leidy, I have been enabled to 

 study better specimens than any heretofore examined, and thus 

 discovered the anomalous character of the young shell. 



P. Americanus seems to be solitary in its habits ; at least, no 

 masses of this species have ever been obtained. In external form 

 it is not unlike the tube of Gastrochsena, and I thus described it 

 at first. The Tennessee specimens, preserved in a soft matrix, 

 show that internally it has the same imperforate septa, convex 

 towards the broader end of the tube, and placed at irregular dis- 

 tances. 



The structure of the 3 r oung shell places this genus, as the t} T pe 

 of a new family, Polorthid^e, among the Cephalopods, and it only 



