346 GEOLOGICAL SURVEY OF TEXAS. 



In the Genera of Cephalopods I used the name of Ryckholt's Asymp- 

 toceras* for this same group, of which the type was Naut. cyclostomus, Phill. 

 If Meek's reasoning holds good it seems to us that both the name Crypto- 

 ceras and Solenocheilus should be dropped in favor of Asymptoceras. The 

 whorls increase very rapidly in all their diameters, and the living chambers 

 are corellatively short. The sides and venter are usually gibbous; the dorsum 

 has either no impressed zone or only a very narrow zone of depression, show- 

 ing how recent was the derivation of this group from the parent gyroceran 

 forms. The siphon is so near the venter that it interrupts the suture in most 

 species. So far as I have been able to see, however, it is to be noted that the 

 edges of the suture do not bend backwards to form a siphonal lobe similar to 

 that of an Ammonoid. The siphon may become central in some adults, as in 

 Asympt. crassiventer. The elliptical form of the young whorl, the large um- 

 bilical perforation, the simple, fine, smooth longitudinal ridges of the whorl 

 in the young, and the presence of abrupt umbilical shoulders, indicate deri- 

 vation from the open whorled form, Aipoceras. The sutures have broad 

 ventral, lateral, and dorsal inflections or lobes, and small annular lobes. 



The European species so far as now known to me are Asympt. dorsale, sp. 

 Phill, crassiventer, sp. De Kon., normale, sp. De Kon., latiseptatum, sp. De 

 Kon., cyclostomum, sp. Phill. and all of them are from the Carboniferous. 

 Asympt. Springeri, sp. White and St. John, capax, sp. Meek and Worthen, 

 and the following, are all that are known to me in this country, all three 

 being also Carboniferous, Coal Measures. 



Asymptoceras Newloni, n. s. 



Loc, Oswego, Kansas, Coal Measures. 



Coll. Nat. Mus., Dr. Newlon. 



Figs. 48, 49, natural size. 



The species in hand is a fragment very similar to As. {Cryptoceras) capax, 

 Meek and Worthen. j- There are three air chambers incompletely preserved 

 in the cast. The last two sutures are 17 mm. apart on the venter. The in- 

 crease in size is very rapid, being as much as 46 mm. in the greatest transverse 

 diameter to 68 mm., a difference of 22 mm. in a distance of only 51 mm., as 



tions oi the sides and abdomen, and the sutures have broad ventral, lateral, and dorsal lobes. 

 The Devonian forms of Temnocheilus, so far as known, have no annular lobe in the centre 

 of the dorsal suture, but this is present in some Carboniferous species like Tern, latus, De- 

 Kon. (Calc. Carb., PL 24, Fig. 2). The siphon, also, is near. the venter in Devonian forms, 

 but shifts nearer to the centre in some Carboniferous species, like Tern, latus. This organ, 

 however, does not approach the periphery near enough to interrupt the line of suture on the 

 venter in any species. 



*Notice sur le Asympt. et Vestin, 1852. 



fGeol. 111., VI, p. 532, PL 33, Fig. 1. 



