346
In the Genera of Cephalopods I used the name of Ryckholt's Asymptoceras 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 Cryptoceras 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, showing 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 inAsympt. crassiventer. The elliptical form of the young whorl, the large umbilical perforation, the simple, fine, smooth longitudinal ridges of the whorl in the young, and the presence of abrupt umbilical shoulders, indicate derivation 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. There are three air chambers incompletely preserved in the cast. The last two sutures are 17 mm. apart on the venter. The increase 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 "
Notice sur le Asympt. et Vestin, 1852.
Geol. Ill., VI, p. 532, Pl. 33, Fig. 1.