94 University of Texas Bulletin 



tion of volution roughly oval, venter smooth in young individuals and 

 very slightly convex. In older individuals, however, there is a median 

 broad, low, very rounded swelling, similar to a median tubercle. In young 

 individuals the ribs become obsolete just ventral to the ventral marginal 

 tubercle, but in older individuals the ribs are continuous across the venter. 

 Ventro-marginal tubercles acute, one on each rib, becoming obsolete in 

 the later volutions. Ribs long and short, mainly unbranched. Dorso- 

 lateral (umbilical) tubercles few and scattered. Cross-section of earlier 

 whorls subrectangular, flanks straight, venter lightly convex, dorsum very 

 concave; in later whorls the flanks are more rounded and make an even 

 curve with the venter. In certain young individuals the ribs are flexuous, 

 having a prominent sigmoidal curve, and are much thickened at the mar- 

 ginal tubercle. This individual shows no ribs at a diameter less than 

 3.8 mm. After that diameter, the intercalated ribs, one or two between 

 some branches, are similar to those of the later volutions. 



The suture of the species resembles that of young individuals of 

 A. martimpreyi Coquand, as figured by Pervinquiere. 1 The considerable 



Fig. 13. Acanthoceras worthense n. sp., suture, camera lucida drawing, x 8. 



amount of material at hand does not contain any sutures which are much 

 more complicated than the ones figured here. The suture shows exter- 

 nally besides the siphonal lobe, four saddles and four lobes, which pro- 

 gressively decrease in size toward the anti-siphonal region. The bifid 

 first saddle is more irregular and less square than in the examples of 

 A. martimpreyi mentioned. All the remaining saddles are simple, or 

 bifid, and the lobes are generally bifid. The first lobe is as tall as the first 



iPervinquiere, Et. pal. tun., p. 294, figs. 109-110. 



