262 PROCEEDINGS OF THE ACADEMY OF 



like those of an oyster, connected by a faint polished line, marking 

 the mantle margin. 



Having pointed out what I believe to be the relations of this 

 remarkable fossil, it yet remains for me to remark on the anoma- 

 lous circumstances connected with the manner of growth of the 

 shell in its later staq-es. 



After growing to a length of nearly an inch, the animals (at 

 least of P. tibialis) seem to have congregated into colonies. 

 Thenceforward the septate character, above described, ceases. 

 The tube increases gradually in diameter as it grows in length, 

 and at distances varying without any determinate system, from a 

 quarter of an inch to two inches apart, the tube is hermetically 

 closed bj r arched septa, with their convex faces towards the mouth 

 of the tube ; or in a reversed position to that of the primary 

 series. The tube consequently could have been of no further use 

 as a float, like that of the other chambered cephalopods, and 

 would have been really disadvantageous to its occupant, was it 

 not permanently anchored as we see it actualby was. The colonies 

 must consequently have resembled a highly magnified view of a 

 bryozoon or a coral. 



EXPLANATION OF PLATE 8. 



Fig. 1. View of a mass of P. tibialis, natural size. 



" 2. One tube, slightly magnified : , muscular scar ; b, secondary septa. 



" 3. An unusually large tube, abnormally bent, natural size: a, one of 

 the secondary septa. 



" 4. Magnified view of end of a tube : a, surface markings of the species 

 (P. tibialis) ; 5, end view of muscular scar on the cast ; c, in- 

 ternal cast of the last chamber and part of the mould of the cen- 

 tral tube. 



" 5. Side view of the end of another specimen : a, a, parts of the shell ; 

 b, b', muscular scar, b seems to correspond with the broad mus- 

 cle of Nautilus ; c, c, primary septa ; d, side view corresponding 

 with c, fig. 4. 



" 6. Top view of a secondary septum broken through at a. 



" 7. End view of one of the primary septa. 



" 8. P. Americanus, natural size. 



und der Ammoniden ; PalaxraUgraphica. Band 17,1870, p. 185, pi. 39 

 and 40. 



