338 GEOLOGICAL SURVEY OF TEXAS. 



the dorsum are broad, smooth, slightly concave zones, which are correlative 

 with the prominence of the shoulders, and are therefore found only in the 

 later stages. The zone of impression along the area of contact between the 

 whorls is narrower and shallow, and probably confined to the later stages of 

 growth; such facts, however, were not determinable with certainty. 



The sutures had saddles at the umbilical shoulders, lateral lobes with slight 

 saddles at the angles of the sides and abdomen, and a deep lobe on the abdo- 

 men. The side view (Fig. 37) is a restoration so far as the internal parts of 

 the umbilicus are concerned, and it makes the young whorls probably flatter 

 on the sides and more like those of the adult than they really were. The front 

 view is in a measure also a restoration, all specimens being more or less im- 

 pressed. A specimen from Texas exists in Prof. Newberry's collection which 

 gives the sutures. 



It is like Metacoceras Sangamonensis, Meek and Worthen,* from the Coal 

 Measures of Illinois; but that species had very nearly the same form as the 

 adult of this species at an age when this still had an embryonic rounded whorl. 

 It may be the same as the Discites tuberculatus, Owen, j but the figure given of 

 this last is too imperfect to admit of comparison. It resembles also the un- 

 named nautilus described by McChesney,J but this shell is much broader 

 transversely and like Sangamonensis matures much earlier, and the abdomen is 

 also hollow like the Metacoceras tuberosus, McCoy. || This last species is more 

 like our M. Walcotti than any other, but differs in the hollowness of the abdo- 

 men and the less prominence of the umbilical shoulders. 



There is a compressed specimen from the coal measures of Kansas, collected 

 by Dr. W. S. Newlon, in the Coll. National Museum, which although much 

 compressed has similar nodes ana lateral ridges and is probably a variety of 

 the same species. 



The species differs from M. Dubium in the earlier stages, the adult char- 

 acteristics being developed very late in the life of the shell, the interior 

 whorls being rounded at a stage when in M. Dubium the full adult characters 

 have been developed. M. Dubium is also broader transversely, and the lines 

 of growth have only a shallow abdominal sinus, whereas in Walcotti there is 

 a sinus of great depth, as in Temnochelius crassus. 



The resemblance to Metacoceras (Nautilus (Gyroceras}), subquadrangularis, 

 Whitefield,§ are more remote. This shell is distinctly gyroceran in its mode of 

 coiling and the umbilical perforation is much larger and young whorls more 



*Geol.Ill., II, p. 386, PL 29, Fig. 3. 



f Geol. Wis., Iowa, and Minn., p. 581, PI. 5, Fig. 14. 



% Trans. Chic. Acad., I, PI. 3, Fig. 6. 



|| British Pal. Rocks, PI. 3?, Fig. 15. 



§ Amer. N. Y. Academy Science", II, No. 8, p. 232, Paleontology Ohio, III, vol. II, p. 16. 



