CARBONIFEROUS CEPHALOPODS. 337 



slightly convex zones on either side of the umbilical shoulders. The umbil- 

 ical shoulders or crests may be slightly nodular in some specimens. There is 

 an impressed zone on the dorsum, the involution embracing the surface of the 

 abdomen but not covering the nodes. The transverse diameter through the 

 umbilical shoulders is greater than that measured through the dorsal part be- 

 tween any two nodes in a full grown shell which has not been distorted by 

 compression. Fig. 35. 



The sutures are nearly straight or slightly concave on the inside of the 

 umbilical shoulders, with a shallow lobe on the dorsal zone, slight lateral lobes 

 on the sides, and small saddles at the junction (genicular crest) of sides and 

 abdomen. On the abdomen there are only very shallow broad lobes. 



Siphuncle above the centre. Living chamber somewhat over one-half of a 

 volution in length and still imperfect. 



Metacoceras Walcottt, n. s. 



Loc. San Saba County, Texas. 



Coll. National Museum. Collected by Dr. Newberry. 



Figs. 36, 37, natural size. 



This species is remarkable for the rapid increase of the abdomino-dorsal 

 diameter of the whorl by growth. The diameter of the largest specimen is 

 167 mm. The dorso-abdominal diameter of the whorl at the beginning of 

 the last quarter of the last volution is about 52 mm., and the same diameter 

 of the corresponding part of the proximate inner whorl does not appear more 

 than half as great, 22 mm. Tne transverse diameter through the large outer 

 row of tubercles is apparently about the same as that through the umbilical 

 shoulder in the last part of the last whorl. At younger stages the whorl is evi- 

 dently much less heavily tuberculated and the transverse section is probably 

 a more or less flattened oval, with the longest diameter transverse instead of 

 vertical as it finally becomes at older stages. It seems doubtful whether the 

 dorsum and abdomen change much in their relations, except in so far as they 

 are altered by the rapid growth of tubercles and umbilical shoulders. The 

 living chamber is above one-half and may have been three-quarters to nearly 

 a full volution in length. The shell is thick, measuring 2 mm. in thickness 

 on the abdomen and somewhat less on the sides. Situation of siphuncle un- 

 known. The abdomen is evenly flattened or convex, and the tubercles are 

 on the edges of the abdomen. The elongation, prominence, and linear ar- 

 rangement of these form nodose ridges. The sides are concave between these 

 and the umbilical shoulders. The umbilical shoulders are very prominent, 

 and form continuous broad ridges with occasional, though very obscure, no- 

 dosities where ribs springing from the genicular nodosities (outer row) cross 



the sides of the whorls. Between these shoulders and the impressed zone of 

 29 — geol. 



