Nautiloidea. 



23 



Among other straight shells may be mentioned JEndoceras^ Actino- 



Actinoceras, and Huronia. The differences between the two former 



... GALLEHY 



and Orthoceras are illustrated by the fine series of specimens in the ^n 



glass case mounted on the wall on the left-hand side of the doorway, wall-cases 



43«. * 7 & 8. 



43*. 



Fig. 43a. — Fragment of weathered specimen of Actinoceras Bigshyi, Bronn, 

 showing the foramina (/) in the walls of the siphuncle, by which the 

 tubuli thrown out by the endosiphon may have communicated with the 

 septal chambers ; (s) the septa. (Nat. size.) From the Cincinnati 

 Group (Ordovician) : Versailles, Kentucky, U.S. 



Fig. 43^. — Ideal section (restored) of Actinoceras, showing beaded siphuncle 

 and septa. 



In Orthoceras the siphuncle is narrow, cylindrical, and usually 

 central or nearly so ; in Actinoceras the portions of the siphuncle 

 between the septa are inflated and bead-like ; while in Endoceras 

 the siphuncle is relatively very large and marginal. Some of 

 the shells of Actinoceras must have been immense, probably ex- 

 ceeding eight feet in length, for in Wall-case 7 may be seen the 

 body-chamber of Actinoceras giganteum, which has a diameter 

 of about 11 inches; and a huge fragment, the whole of which 

 appears to be septate, measures 2 ft. 5 in. in length, the diameter 

 of the larger end being about 8 inches, that of the smaller about 

 4^ inches. This genus, which possibly occurs in the Cambrian, 

 ranges through the Ordovician, Silurian, and Devonian to the 

 Carboniferous. 



Huronia resembles Actinoceras, but the portions of the siphuncle 

 between the septa are more inflated in their anterior than in their 



