CLINTON GROUP. 



95 



closer together, and the margins are extended or projecting in the cast, which is not observable 

 iti that species. Many of the specimens are weathered, exhibiting the structure in a very 

 beautiful manner, and showing also the variable aspects under which the same species may 

 be presented. 



Fig. 1 a. A fragment of the cast of a specimen, having the edges of the septa slightly projecting. 



Fig. 1 h. Longitudinal section, showing the arching of the septa and form of the siphuncle. 



Fig. 1 c. A fragment where the laminae of the shell have become silicified and separated, so 

 that the septa and siphuncle appear to be composed of double laminse. The interior 

 of the siphuncle is hollow in this specimen. 



Fig. \ d. K fragment of the siphuncle, and several of the septa attached. The siphuncle is 

 worn down in part, exhibiting a section having the lamina thickened, while other 

 portions retain their entire character. 



Fig. 1 e. A fragment of the siphuncle, with parts of the septa attached. At the lower part the 

 shell is removed, showing a cast of the interior of the siphuncle, having the con- 

 tractions and expansions corresponding to those of the outer covering. 



Fig 1/. A fragment having the walls of the siphuncle broken down, and the cast of the interior 

 standing out very prominently. 



Fig. 1 ^. A portion of the cast of the siphuncle enlarged, showing the rugas impressed upon it 

 by the interior rugose surface of the siphuncle. 



The specimen fig. 1 y, exhibits in a very satisfactory manner the character of those specimens 

 where the shell is entirely removed, and casts of the cavities only are left. In such specimens 

 the characters of the Genus Ormoceras may be overlooked, and the true relations of the species 

 misunderstood, or referred to the Genus Actinoceras. 



I have termed the interior filling of these siphuncles casts., because I do not see any organic 

 character upon their surfaces or in the substance, neither is there any evidence that they were 

 originally connected with the walls of the siphuncle. They present upon their surface the rugae 

 corresponding to the rugae shown in the siphuncle of Ormoceras (Pal. N. York, Vol. i, pi. 

 17, fig. 1 a), and there are no rays uniting with the walls of the siphuncle. It is true that in 

 some specimens where the expanded portion of the interior does not reach the walls of the 

 siphon, there are verticillations of crystalline or semi-crystalline matter extending to the interior 

 wall of the siphon. In good specimens, it is readily seen that there is no organic connexion ; 

 for the expansion is equal on all sides, except that where perfect, the thin edge of the cast is 

 crenulated from the rugae as shown in fig. 1 g-, pi. 29. 



In order to exhibit this " tube" or cast of the interior, a peculiar condition of the fossil is 

 required, as is shown in a comparison of the specimens here represented ; since in fig. 1 h 

 there is no evidence of its existence, and scarcely at all in 1 c and 1 d. In the specimen 1 e, 

 the continuation of the upper extremity shows this cast in a very different state from what it 

 is in the lower part of the same. In all the specimens from the Black-river limestone, where 

 the rock is calcareous, there is no interior tube or cast, though the siphuncles are preserved in 

 every possible condition. In the present species, which in other respects is very similar to that 



