TRENTON LIMESTONE. 219 



283. 15. ENDOCERAS APPROXIMATUM (n. 5p.). 



Pl. LIV. Fig. 2 a. 



Cylindrical, gradually tapering ; septa with a convexity little more than one fourth their 

 diameter, distant about one fifth the diameter of the shell ; siphuncle large, marginal, 

 obliquely annulated by the thin edges of the septa. 



This specimen is a fragment, preserving about five or six chambers, with a portion of the 

 siphuncle projecting above. 



In many respects, this species resembles the E. proteiforme ; but the i^arkings of the 

 septa upon the siphuncle are more approximate in proportion to its diameter. In the one 

 figured, they are distant about one third the diameter of the siphuncle ; being actually 

 nearer than those ofsmaller siphuncles of that species figured on Plate L, which are distant 

 two fifths of the diameter of the tube. 



There is, however, still some obscurity in regard to these parts of the shell, and we are 

 not able to decide whether their proportions are constant. This earsonly be determined by 

 a comparison of all the parts of a large number of specimens of authentic species. I have 

 examined three specimens of what appear to be, very satisfactorily, the siphuncles of E. 

 proteiforme, two of them containing a portion of the embryo tube. In all these the diameter 

 is almost precisely the same, and the distance of the septa corresponds. On this account, I 

 have thought it preferable to indicate this one as a distinct but approximating species. 



Position and locality. This species is found, associated with the preceding, in the higher 

 part of the Trenton limestone near Middleville. 



284. 16. ENDOCERAS DUPLICATUM (n. 5JJ.). 

 Pl. LV. Fig. 1. 



Cylindrical, gradually tapering ; septa distant about three eighths of an inch ; siphuncle 

 extremely distended, and containing two embryo tubes, within each of which is a young 

 shell. 



The only specimen seen is much compressed, and distorted so that its original form can 

 not be made out. It evidently contains two embryo tubes, which apparently belong to the 

 specimen. This is quite distinct from the Endoceras gemelliparum of the Black-river lime- 

 stone, both in its general form, distance of septa and character of the tubes, showing a 

 second species with double embryo sheaths. 



Position and locality. This species occurs in the upper shaly part of the Trenton limestone 

 near Middleville. (Cabinet of John Gebhard, Jr.) 



28' 



