348 GEOLOGICAL SURVEY OF TEXAS. 



are near the umbilici, and from this part the whorl is slightly concave on 

 both sides towards the periphery or abdomen. This although very narrow is 

 flattened or slightly convex, even in the largest specimens. 



The living chamber in one specimen was about one-half of a volution in 

 length. The lines of growth* indicate that the aperture probably had very 

 broad lateral saddles and a single deep, narrow median abdominal lobe. 



The sutures are near each other or slightly crowded in aspect. They have 

 a narrow abdominal saddle, deep, broad lateral lobes, comparatively narrow 

 lateral saddles near the umbilici, and a pair of shallow lateral lobes internally 

 on the shoulders of the whorls. 



The shell is thin and it is marked by fine lines of growth. The siphuncle 

 is probably situated near the abdomen, but was not clearly seen. 



A specimen sent me by Mr. Hay from Fort Riley is the most perfect speci- 

 men of this remarkable species that I have yet seen. It has an almost entire 

 living chamber about one-half of a volution in length, the sutures show well, 

 and it is not as much compressed as specimens from Texas. All the speci- 

 mens are reported as coming from Carboniferous, as do all species of the genus 

 so far found. 



The sutures may have a slight lobe on the hollow of the narrow abdomen, 

 where compression has affected them ; where they are unaffected by compres- 

 sion they are absolutely straight or very faintly concave. In Mr. Hay's cast 

 the outer part of the living chamber presents the abdomen as slightly convex, 

 and leads one to think that the slight hollowness of the abdomen often present 

 in younger whorls is due to compression. In fact the whorl is broken along 

 a line parallel with and near to the edge of the abdomen and is concave from 

 compression on the right hand (morphologically left) side until near the end 

 of the living chamber. Here, where the abdomen presents a very flat con- 

 vex surface, both sides of the whorl are unbroken and have the normal pro- 

 portions. Figure 28 is therefore in part a restoration. 



This is the largest and finest species of the involute shells of this group yet 

 found in the Carboniferous. The principal differences between it and Nauti- 

 lus Rouilleri, the adult of which was described and figured by Trautschold f 

 under the name of Oxystomus, £ and the young by Marie Tzwetaev,|| consist 

 in its size. The principal difference between the European and American is, 



* The lines of growth in the drawing have the first lateral saddles or inflections too prom- 

 inent and the second pair not prominent enough, the lobe between being too deep. 



f Kalbruche von Miatschkowo, p. 28, PL 3, Fig. 7. 



JThe name Rouilleri was given to this as the type in De Koninck's Calcaire Carbonifere, 

 p. 124, in his description of Nautilus Oxystomus, which last was afterwards taken by the 

 writer as the type of his genus Phacoceras in Genera of Fossil Cephalopods. (Proc. Bost. 

 Soc. Nat. Hist, XXII, 1883, p. 292.) 



|| Op. cat, p. 53, PL 6, Figs. 33-34. 



