TRENTON LIMESTONE. 
213 
septa, figured on PI. XLVII, figs. 4 a, b , c, d , the siphuncle is nearly central, and, in the 
smallest one, it scarcely deviates from that position. This character, therefore, is not to be 
regarded as of primary importance. The dimension of the siphuncles in the two varieties 
is precisely similar in specimens of equal size. 
In the figures given, one or two of the last septa are represented as more approximate 
than the other. This character is likewise common to other species, and, I believe, to nearly 
all those where I have had an opportunity of examining this portion of the shell. 
In the variable character of the striae, and the crowding together in bands, this one 
presents all the variations observed in the preceding variety. 
Fig. 4 a. A fragment, showing the contraction below the aperture. 
Fig. 4 b. Transverse section, showing the nearly central position of the siphuncle. 
Fig. 4 c. A portion of the surface of a specimen enlarged. 
Fig. 4 d, e. Two specimens denuded of the shell, showing a uniform character in the contraction of the 
tube, with a slight difference in the distance of the septa. 
ENDOCERAS PROTEIFORME. 
ILLUSTRATIONS OF THE OLD SHELLS, WITH THE EMBRYO SHEATHS OR TUBES SEPARATE 
AND ENCLOSED WITHIN THE SIPHUNCLE, CtC. 
Pl. XLVI. Figs. 1 a,b; Pn. XLVIII. Figs. 1, 2, 3, 4; Pl. XLIX. Figs. 1 a-e; Pl. L. Figs. 1, 2, 3; and 
Pl. LIII. Fig. 2. 
These figures illustrate some of the many different forms in which this species occurs. 
They further show that the smooth embryo tube or sheath is almost constantly present 
within the siphuncle, as well as often separated from it. In all the specimens figured, it 
will be observed that the apex of this tube is directed towards the apex of the parent shell, 
thus precluding the idea of its accidental occurrence in this situation and position. The 
uniformly smooth surface of these tubes, which have never been found to contain septa or 
siphuncle, while on the other hand they contain septate tubes with peculiar surface 
markings, indicates that they performed some other function, and probably only changed 
their position by accident, or the death of the enclosing animal, and consequent destruction 
of its habitation. We shall perceive, from the evidences to follow, that the young shell of 
this species, at least, was enclosed during its early stages, and probably always until fully 
developed, within a shelly tube. 
Plate XLVI. 
Fig. 1 a , b. This specimen has been noticed before, as containing a young shell of the var. Ihieolatum. 
The parent shell, or cast, has the septa distant about one fifth or one sixth the diameter of 
the shell. The embryo tube is smooth, slightly projecting at one extremity, and containing 
a smaller striated tube. The specimen is crushed, so that the section is imperfect. 
