Cephalopoda of the Cincinnati Group. Vl'-\ 



No appearance of muscular scars have ever been noticed on the 

 oast or the shell of the body chamber of any specimen of this cla.'sp 

 found in our rocks. I have seen a great many good casts of the body 

 chamber of Oiihoceras, and some good shells of the body chamber, 

 but none of them have shown the least indication of muscular scart*. 

 From these facts I have received the impression, that the siphuncle 

 alone was the point of muscular attachment. If this view is correct, 

 then the embryonic state of the Nautilus would seem to be the rep- 

 resentative of the full developed Cchhalopods of the Silurian age. 



It appears, too, from an examination of some specimens, that several 

 >iepta were forming at the same time in the body chamber, the 

 anterior ones being only rudimentary, while, posteriorly, they ap- 

 proached nearer and nearer to the siphuncle, or point of muscular 

 attachment. 



The method of growth would seem to have been as follows : each 

 septum began to form at the circumference of the shell, near the 

 anterior part of the body chamber, to accommodate the on ward growth 

 of the shell, and slowly approached the siphuncle, as it moved for- 

 ward from septum to septum ; the siphuncle being a point of muscular 

 attachment, was not vacated by the animal between any two septa, 

 until the anterior one had been firmly closed by attachment to the 

 it^iphuncle, forming a chamber of support. The chamber, thus cut off, 

 afterward filled more or less with water, that found its way through 

 the pores of the shell, but otherwise remained firmly ch)sed. For 

 this reason the chambers of the Orihoccras are usually filled with 

 crystalline carbonate of lime, while the siphuncle and body chamber 

 are filled with minute fossils, fragments of shells, or uncrystalline 

 matter. 



It remains now to be said, that the shells of this class, in our rocks, 

 are very thin in j^roportion to their size, and that there is nothing to 

 indicate that the animals were powerful or carnivorous, though they 

 might have been carnivorous. The character they have received of 

 " mon!>ters vast of ages past," is not supported by investigation. 

 They were strong enough to sustain themselves among the much 

 smaller animals, with Avhich they associated, and that is the most that 

 can be said of their power. 



The Orthoceratidm and the Cyrtoceratidce are the only tAvo fiimilioP 

 belonging to the Tetrabranchi'ita, represented in the Cincinnati Group, 

 about which there seems to be no doubt. Prof i\leek, however, de- 

 scribed a fossil found somewhere about Richmond, Ind., as TrochoceruK 

 JBaerL If he is right iu his genus, it adds to our rocks the family 

 Trochoceratidie, a family not before known to exist in the Lower 



