TKENTON LIMESTONE. 213 



septa, figured on PL XL VII, 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 siph uncles 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 frag-ment, showing the contraction below the aperture. 

 Fig. 4 h. Transverse section, showing the nearly central position of the siphuncle. 

 Fig. 4 e. A portion of the surface of a specimen enlarged- 

 Fig. 4 d, e. Two specimens denuded of the shell, showing a uniform chafracter 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. I a,b; Pl. XLVIH. Figs. 1, 2, 3, 4; Pl. XLIX. Figs. 1 a-e; Pl. L. Figs. 1, 2, 3; and 



Pi.. 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 XLVL 



Fig. 1 a, b. This specimen has been noticed before, as containing a young shell of the var. lineolatum. 

 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. 



