148 University of Texas Bulletin 



metrical. The rest of the saddles and lobes (near the umbilicus) are 

 simple, asymmetrical undulations. 



The suture is nearly straight, slightly curved toward the front, with 

 an inflection backward in the siphonal and antisiphonal regions. 



Vidrioceras resembles in many respects Marathonites. The prin- 

 cipal difiference is to be found in the details of the suture line. The 

 siphonal and antisiphonal lobes are very similar in both subgenera, 

 but while Marathonites has nine lobes and eight saddles on the internal 

 side, Vidrioceras has only seven lobes and eight saddles ; and while the 

 former has one lobe on the umbilical wall, the latter has two. Another 

 difference is to be found in the shape of the first auxiliary saddle. In 

 Marathonites. this is very similar to the external and lateral, while 

 in Vidrioceras, it is of the common Glyphioceras type; and while in 

 the first-mentioned subgenus the second auxiliary lobe is decidedly 

 bifid, in the other subgenus it is a simple and very shallow notch be- 

 tween tv/o auxiliary saddles. The greatest difiference, perhaps, exists 

 in the form of the lobes, which in Vidrioceras never become really 

 trifid. 



On account of the small number of specimens I have not been able 

 to study the entire development of the suture in Vidrioceras, but in 

 a specimen of about 7.5 mm. diameter, I could trace the suture in the 

 lower half. It is much simpler than in the larger whorls. The 

 branches of the siphonal lobe end in one point, being still very asym- 

 metrical, but the first lateral lobe is practically rounded and has lost 

 its two points. The same is the case with the first auxiliary lobe, 

 while the second lateral shows still its only sharp point. The external 

 saddle is much higher- than the two laterals, the first lateral is very 

 low and narrow, while the second one is about as high as the first 

 one, but much broader. The first auxiliary saddle is much higher 

 than the second lateral, but is about as broad. The second auxiliary 

 lobe disappears almost entirely. 



When we compare this suture with that of a Marathonites vidricn- 

 sis of about the same size, we find that in the latter one the suture 

 is still entirely the same as in the larger whorls. 



