MENOCEPHALUS AND ASAPHUS IOWENSIS. 577 
larger of the two has the glabella less acuminated in front, and more rectangular, and the facial sutures 
more sinuous; but the distinctive specific characters of the two cannot be well established until more 
complete specimens are found. 
MENOCEPHALUS. (N. G.) 
Figure 11 of Tab. I. is the glabella and a small portion of the cheek-plate of a Trilobite, which will, no 
doubt, constitute a new genus, for which the name Menocephalus is proposed. 
The glabella of this Trilobite differs from that of all the preceding in its circular, highly arched, and 
hemispherical form, and in being pustulated. Except this very characteristic form of the glabella,.no 
other remains have yet been discovered that can be referred with any degree of probability to this genus, 
a further description of which must be deferred until the labours of the palzontologist can disclose more 
complete specimens of this highly interesting Crustacean. 
ASAPHUS (ISOTELUS) IOWENSIS. 
(Tab. IL. a, figs. 1,2, 8, 4, 5, 6, 7.) 
According to Burmeister, Hawle, and Corda, this Trilobite would fall into the genus Asaphus, inasmuch 
as they consider that that genus has but eight thoracic segments ; according to other authors, into the genus 
Asotelus, which, however, is not recognised as a distinct genus by the above writers on Trilobites. 
The general form and contour of the cephalic shield closely resemble that of Asaphus platycephalus of 
Stokes (Jsotelus gigas of Dekay); but the facial sutures do not converge in front to form a distinct angle, 
but describe three parts of an almost regular circle, as in Asaphus expansus* (Hemicrypterus, according 
to Hawle and Corda), as shown by fig. 4, Tab. II., and fig. 5 of Tab. II. a, inadvertently inverted in posi- 
tion by the engraver. 
Relations and differences.—This Trilobite stands as a connecting link between Asaphus, Phacops, Hemi- 
erypterus, and Isotelus. Like many of the species of Phacops, it is provided with spines. The facial 
sutures do not form, as has been remarked, a distinct angle in front of the glabella, as in Isotelus gigas, 
but part of a nearly regular circle, as in Hemicrypterus. Like Asaphus, the eyes are reticulated, and the 
middle lobe of the caudal shield is defined (though sometimes somewhat indistinctly), but the segments 
are only obscurely pronounced. The axal lobe of the thorax is not as wide as in Jsotelus, nor yet as narrow 
as in most species of Asaphus. Like Isotelus, the glabella is but obscurely defined. 
From Dr. Locke’s megistos, it differs in the eyes being set closer together; in the spines being longer— 
extending as low as the caudal shield; the pygidium more regularly elliptical; and its axal lobe more 
distinctly defined. 
On Turkey River, in Iowa, where this species occurs, it seems never to have attained the great size 
which Dr. Locke’s megistos did in Ohio; the largest specimens obtained at Turkey River are not quite 
four inches long. These differences we consider sufficient for the establishment of a new species. The 
uniform presence of spines at the angles of the wing in this Towa fossil, as well as in the species found 
by Dr. Locke in the Blue Limestone of Ohio, and described by him under the specific name megistos, 
separates them from the gigas of Dekay, unless this can be shown to be a distinction of sex only. 
CEPHALOPODA. 
AMMONITES NEBRASCENSIS. (N. 8S.) 
(Tab. VIIL., fig. 3, and Tab. VIII. a, fig. 2.) 
Specific character.—Discoidal, slightly convex, whorls one, and part of a second concealed in the ter- 
minal portion; flattened on the dorsum; surface ornamented with distinct, tuberculated ribs, every 
second or third of which is somewhat more prominent than the intermediate ones, which latter are thrown 
* Some of the figures given by Hall of what he regards as J. gigas are but very slightly angled, and almost approach 
an arch. (See Paleontology of New York, Pl. 61, fig. 3c, and f) In all essential features, the Isotelus agrees 
with Asaphus; and if Burmeister is right in giving to this genus but eight segments, instead of nine, as previous 
authors have done, then the genus Jsotelus must be abandoned. 
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