CARBONIFEROUS CEPHALOPODS. 341 



the sides are less conspicuous upon this cast. The sutures have about the 

 same general contour as in the nearest ally just mentioned, but the lateral 

 lobes are broader and shallower and the saddles at the umbilical shoulders are 

 not so prominent. s- >w 



The young do not seem to have the pilse so plainly shown 

 in the umbilicus of Metac. cavatiforrnis, but the cast may de- 

 ceive the observer in this respect. 



Fig. 41 represents a section of the adolescent whorl with- 

 out tubercles. ~T?ig. 41. 



Tainoceras cavatum, n. s. 



Loc. Texas. 



Geol. Surv. of Texas. 



Fig. 42-44, natural size. 



The cast of this shell has distinctly marked lines of nodes, two on each 

 side and two on the abdomen (outer side). The umbilical shoulders of the 

 whorls are very broad but slightly convex and divergent. The umbilici are 

 consequently deep and broadly coniform. The sides are flat, narrow, and 

 not as broad as the umbilical shoulders of the whorl. The abdomen is very 

 much broader than the dorsum and consists of three longitudinal divisions, 

 a smooth zone on either side lying between the outer lateral and the proxi- 

 mate abdominal row of nodes. These two internodal zones are only very 

 slightly convex, have no ribs, and the median zone lying between them is also 

 free of ribs and decidedly concave. No shell was seen. The sutures are 

 moderately closely set. The living chamber, of which the larger part is pre- 

 served, was probably, judging from markings on the cast, not less than half 

 a volution in length. The increase by growth in the lateral transverse diam- 

 eters is much more rapid than in the dorso-ventral diameters of the whorl, 

 and the last whorl therefore grows broader quite rapidly. Siphuncle was not 

 seen. 



The smaller specimen of the two under examination is also a cast, but it 

 shows the umbilicus quite plainly. This is deep, and the narrow, flattened 

 sides of the later stages arise on the latter part of the second or the first 

 quarter of the third volution.* The second volution has a very broad abdo- 

 men and convex sides dipping steeply towards the funnel-shaped umbilici as 

 in Temnocheilus. Whether the sides had one row of tubercles along the crests 

 at the junctions of the abdomen and sides before these began to spread out 

 to form the flattened sides of the latter stages could not be determined — none 

 were present on the cast. (Fig. 44.) But it is probable that the other row 



*Number of volutions are estimated; the beginning of the first volution is destroyed in the 

 fossil. 



