354 GEOLOGICAL SURVEY OF TEXAS. 



the abrupt and prominent umbilical shoulders by narrow uncovered zones. 

 The young have whorls with transverse diameter more than twice as broad 

 as the abdomino-dorsal, at least in the nealogic stages. At diameter of 37 

 mm. the whorl is as 13 mm. in abdomino-dorsal diameter to 27 mm. in trans- 

 verse. The umbilical shoulder stands out from the side of the next inner 

 whorl fully 8 mm. at this same diameter. In consequence of the large and 

 subangular umbilical shoulders and the excessive rate of increase in the trans- 

 verse diameter a whorl is produced which resembles that of Asymptoceras in 

 general aspect. It can be at once distinguished, however, by the position of 

 the siphuncle and the invariable continuity of the ventral suture. 



The living chamber is considerably longer than one would expect in such 

 a broad shell. It is, though incomplete, considerably over one-fourth of a 

 volution in one specimen and in another somewhat longer. It is when com- 

 plete probably in the neighborhood of one-half of a volution in length. 

 The aperture was not clearly seen, but in the largest specimen there is what 

 seems to be wing-like saddles on the sides as continuations of the prominent 

 umbilical shoulders. These increase the resemblance of the terminal or liv- 

 ing chamber portion o*f the whorl to Asymptoceras. 



The sutures are widely separated, being 9 mm. apart at diameter of 27 

 mm. This extreme width of the air chambers is not maintained in full 

 grown specimens, but the sutures are approximate only in outgrown speci- 

 mens. The sutures have very shallow ventral and lateral lobes with corre- 

 spondingly slight saddles at the junction of the sides and abdomen (and 

 probably at the umbilical shoulders), but these are mere inflections, the out- 

 lines being almost straight. The sutures at the umbilical shoulders and 

 inside of these were not seen except in section of the dorsal area. The 

 impressed zone on the dorsal area is deep, and in a section which strikes ob- 

 liquely across the septa a distinct annular cone and small annular lobe are 

 exposed. This cone was well developed in the nealogic stage before the 

 whole shell had reached the diameter of 27 mm. 



GONIATITINAE. 



GASTRIOCERAS. 



The species of this genus have very broad depressed whorls with open 

 and very deep umbilici, but the outlines of the sutures are the tests of affinity, 

 the form of the whorl being the more variable quantity among shell-covered 

 cephalopoda. As I have frequently pointed out, two shells may closely re- 

 semble each other in form and in external characters and yet belong to dis- 

 tinct groups, genera, families, or sub-orders; and it is even possible to make 



