TKENTON LIMESTONE. 209 



siphuiicle. Those young' shells are also frequently foiiiul separate from the parent shell or 

 embryo tube, when we are compelled to rely ujjou the surface markings for their lie- 

 terminalioii. 'I'lie position of the siphuncle, convexity of septa, and some other characters, 

 are usually consiaul in all the varieties, wliich only exhibit a change in the character of 

 the surface. In the aijseuce of septa and sijihuncle, which is of common occtnrence in the 

 young shell, the character of the surface is reliabh; for determining the species. 



274. 6. ENUOCERAS PROTEIFORME, var. TENUISTRTATTiM. 



I'l, XLV. Fics. 1 a,h; and Pi.. XLVII. Figs. 1 a, 4, and 2 a, b, c, iJ, 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. Tiie 

 longitudinal stri;e are not more than one fourth as large as the transverse ones, and visible 

 only under a magnifier ; they often become obsolete and scarcely jjerceplible, and the 

 transverse strias assume a slightly imbricating and lainellose structure. The transverse 

 striifi are often crowded together in bands or fasciae, as shown in the figures. 



In examining other specimens, we find the longitudinal striaj 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 

 inclics 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 wilh 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 she. 

 Fig. 1 b. An enlarged portion, showing the longitudinal stria3. 



Plate XLVII. ^ 



Fig. 1 ffi. A smaller and more slender specimen of the same species. 



Fig. \ 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 strice 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 strio8 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. 

 [PaLvEontologv.J 27 



