TRENTON LIMESTONE. 
209 
siphuncle. These young shells are also frequently found separate from the parent shell or 
embryo tube, when we are compelled to rely upon the surface markings for their de¬ 
termination. The position of the siphuncle, convexity of septa, and some other characters, 
are usually constant in all the varieties, which only exhibit a change in the character of 
the surface. In the absence of septa and siphuncle, which is of common occurrence in the 
young shell, the character of the surface is reliable for determining the species. 
274. 6. ENDOCERAS PROTEIFORME, var. TENUISTRIATUM. 
Pl, XLV. Figs. 1 a, b ; and Pl. XLVII. Figs. 1 a, b, and 2 a, b, c, d , e. 
This variety is known by the character of the surface, which is marked by regular 
transverse striae, sometimes appearing as if imbricated, or having one edge elevated. The 
longitudinal striae are not more than one fourth as large as the transverse ones, and visible 
only under a magnifier ; they often become obsolete and scarcely perceptible, and the 
transverse striae assume a slightly imbricating and lamellose structure. The transverse 
striae are often crowded together in bands or fasciae, as shown in the figures. 
In examining other specimens, we find the longitudinal striae becoming more distinct, 
until finally they are equal to the transverse ones, and the entire aspect of the surface is 
changed. 
All the specimens examined of this variety are more or less imperfect, and, with a single 
exception, disconnected from the parent tube, and destitute of septa. In a specimen five 
inches long, the diameter of the larger end is one inch and a quarter, while that of the 
smaller end is half an inch. In another specimen with a diameter of half an inch at the 
larger extremity, the smaller end is one eighth of an inch in diameter. 
Plate XLV. 
Fig. 1 a. A fragment of the young shell, of the natural size. 
Fig. 1 b. An enlarged portion, showing the longitudinal stria?. 
Plate XLVII. 
Fig. \ a. A smaller and more slender specimen of the same species. 
Fig. 1 b. A portion of the surface enlarged, showing the cancellated lines. 
Fig. 2 a. This specimen presents very similar surface markings, and cannot be distinguished from the 
last, though the transverse striae are less prominent. The specimen consists of a double or 
triple tube; the outer one 2 b showing externally marks of septa, which are distant about 
one fourth the diameter. The shell is exfoliated, and the surface markings are unknown. 
The inner tube, which is shown in the upper part of the figure, is likewise septate in its 
upper part, the septa being at about the same proportional distance as in the last. A portion 
of the shell still remains upon this one, which preserves the markings before described. 
Fig. 2 c. An enlarged portion of the surface; the longitudinal striae are too strong in the figure. 
Fig. 2 d. A small portion still farther magnified, showing the proportionate size of the transverse and 
longitudinal stria?. 
[Palaeontology. J 
27 
