REPTILIAN AGE. JIJIIASSIC PERIOD 123 



lobe is smaller than the lateral saddle, and divided at the extremity into two nearly 

 equal, rather short branches, each of which is sinuous, and shows a disposition to 

 give off short subdivisions on the outer side. The remaining lobes are very small, 

 and obtusely digitate, the inner one showing a tendency to bifurcate. 



This species varies considerably in form, as well as in its surface markings, at 

 different ages ; young specimens being much more compressed, more sharply cari- 

 nate on the dorsum, and having a proportionally wider and more shallow umbihcus 

 than the adult. Their costse are also more flexuous and more sharply elevated than 

 those of mature shells. On medium-sized specimens the costse that pass entirely 

 across the sides sometimes swell a little near the umbUicus, so as to form very 

 obscure, transversely elongated, subnodose prominences; while on large specimens 

 all the costae are nearly or quite obsolete. 



If Ammonites coirlatus, of Sowerby, varies as much as it would seem to do from 

 the figures of it given by D'Orbigny in the Palaeontology of France, and in Mur- 

 chison, De Verneuil, and Keyserling's work on the Geology of Russia, our shell 

 may prove to belong to that species. In form and external ornaments it is almost 

 exactly like some varieties of A. eordatus, yet it presents rather marked differences 

 in its septa from any of the figures of that species we have seen; not greater, 

 however, than we see between the septa of supposed individuals of that extremely 

 variable form, represented by D'Orbigny in the Pal. France, and in the Geol. Russia. 

 Although later comparisons have nearly satisfied us that our shell is not distinct 

 from Sowerby's species, we have concluded to retain our name, cordiformis, until 

 its identity or difference can be determined by the comparison of a better series of 

 specimens. 



Locality and position. — Southwest base of the Black Hills, associated with Belem- 

 nites densus, Eumicrotis curta, and other Jurassic fossils. (Type No. 203.) 



Ammonites IScnryi. 



(Plate IV, Fig. 9, o, h, c,) 

 Ammonites Henryi, Meek & Hatden, Proceed. Acad. Nat. Sci. Phila. March, 1858, 57 ; ib. Oct. 1860, 418. 

 Shell convex-lentiaular ; dorsum narrowly rounded or subangular ; umbilicus very small or nearly closed. Vo- 

 lutions about doubling their diameter every turn ; inner ones entirely hidden within the profound ventral groove 

 of each succeeding whorl. Surface apparently without nodes or costse. 



The septa are rather closely crowded, but their lobes and saddles are not very 

 deeply sinuous, or complex in their subdivisions, and differ somewhat in their 

 details on opposite sides of the shell. None of our specimens are in a condition to 

 show very clearly the whole of the dorsal lobe, though it appears to be as long as 

 the superior lateral lobe, and has at the extremity two small approximate terminal 

 divisions, each of which is provided at the end with three or four small digitations; 

 above these there is on each side one broad, but short, bifurcating, and more or less 

 digitate lateral branch, and, above that, one or two subordinate lateral sinuosities. 

 The dorsal saddle is about as large as the superior lateral lobe, a little oblique, and 

 has at the extremity two short, nearly equal, digitate divisions, each of which shows 

 a tendency to bifurcate. Below these there is on each side one lateral branch. 

 The superior lateral lobe is as large as the dorsal, somewhat obliquely conical, and 

 tripartite at the extremity, the divisions being small, subequal, divergent, and 



