204 University of Texas Bulletin 



lateral lobe, although the ground plan in all the species of the genus 

 is evidently the same, and shows the near relationship between them. 



Evidently we have here another example of development of one 

 tribe in different horizons, the oldest form showing the simplest su- 

 tures, while the youngest have the most complicated. But the differ- 

 ences are much more difficult to describe, because the external form 

 of the species varies very little, and the suture is more uniform, as 

 for example in Medlicottia. The differences are mainly found in the 

 secondary elements of the suture and only very close observations 

 show that a real development exists. There may even be recurrences 

 in higher horizons, as is shown by the relatively simple suture of the 

 Perrinites found in the high horizon of Quanah. 



No Perrinites has been found elsewhere than in Texas; this genus 

 therefore cannot serve to correlate our horizons with those o£ other 

 countries. We shall discuss this point later on, and refer also to 

 what has been said in an earlier chapter. 



Gastrioceras n. sp. 



Shell discoidal, very evolute, whorls very little embracing, generally 

 covering only the ventral part of the next older one, very slowly grow- 

 ing in height in the greater part of the inner whorls. Cross-section 

 of the inner whorls much broader than high, nearly forming a rect- 

 angle or a trapeze with rounded edges; in the larger whorls broader 

 than high, but nearly semi-circular, in the largest whorl apparently 

 trapezoidal. On the inner whorls the flanks are only slightly convex 

 and inclined toward the umbilicus while the venter is flattened, thus 

 causing the cross-section to become somewhat angular. In whorls 

 of medium size, flanks and venter are rather regularly rounded, no 

 edge existing between the border and the flanks. In the largest whorl 

 the flanks are nearly flat, strongly inclining toward the venter which 

 is narrow and slightly rounded. The umbilicus is very wide and shal- 

 low, the umbilical border is strongly rounded and passes in a contin- 

 uous curve into the narrow and not very steep umbilical wall ; only in 

 the largest whorls the umbilical border is more pronounced in conse- 

 quence of the different relative position of the umbilical wall and the 

 flank. 



