INVERTEBRATE PALEONTOLOGY. 400 



being curved on the same plane so as to presenl a compressed-subelliptic 

 general outline; first turns regularly involute, but nut in contact, after which 

 the succeeding portion is deflected more or less nearly straight away from 

 the regular curves for some distance, and then curves back in the form of a 

 hook, without coming in contact with the other parts; surface generally 

 ornamented with annular, often more or less oblique, nodose or spiniferous 

 costae; septa symmetrically divided into six unequal, branching, and digitate 

 lobes, all of which, excepting the siphonal one, are tripartitely subdivided. 



This <renus is nearly related to the elongated section of Scaphi/cs, such 

 as S. Tvanii, Puzos, and S. gigas, Sowerby, from which it only differs in 

 form, by having its first turns coiled so as not to come in contact ; its relations 

 to that genus being exactly the same as Hanrites to Vty choc eras. In imper- 

 fect specimens, showing only the elongated and hooked body-part of these 

 shells, it is difficult to distinguish this genus from Hamltes ; while the regu- 

 larly-coiled portion of the shell must be with equal difficulty distinguished 

 from Crwceratites (usually written Crioceras) of Le'veille". 



The genus Ancyloceras seems to have first appeared during the deposi- 

 tion of the Lower Oolite, and ranges into the Cretaceous, being mainly, or 

 possibly entirely, confined to the lower members of the latter series, where it 

 attains its greatest numerical development. It is entirely unknown in the 

 Tertiaries, or among existing mollusks. 



Ancyloceras! u n c n in , M. & H. 



Plate 21, figs. l,a,b. 

 Ancyloceran (Hamilcs) uncum, Meek and Hayden (1858), Proceed. Acad. Nat. Sci. Philad., X, 56. 



The only specimen of this species yet found is imperfect at both extrem- 

 ities, and consists merely of a portion of the outer chamber. It is a little 

 compressed laterally, and rather abruptly bent in the form of a hook, after 

 which the two extremities seem to be extended in the same direction and on 

 the same plane, with a free space between, equaling near the curve, aboul 

 half the greater diameter of the larger limb. The surface is ornamented by 

 rather distinct, annular costse, which are angular, less than the rounded 

 depressions between, and encircle the shell rather obliquely, being somewhat 

 Hexuous on the sides, and passing straight across the ventral or outer side, 

 where they support two rows of small nodes. The costse are rather obscure 

 on the inner side, near which they sometimes bifurcate at the curve. 



Greater diameter, 1.73 inches; smaller diameter about 1.22 inches. 

 52 ir 



