MENOCBPHALUS 



AND ASAP II US 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. 



MENOCEPIIALUS. (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 palaeontologist can disclose more 

 complete specimens of this highly interesting Crustacean. 



ASAPHUS (ISOTELUS) IOWENSIS. 

 (Tab. II. a, figs. 1, 2, 3, 4, 5, 0, 7.) 



According to Burmcister, Hawle, and Corda, this Trilobite would fall into the genus Asaplius, inasmuch 

 as they consider that that genus has but eight thoracic segments ; according to other authors, into the genus 

 Isotelus, which, however, is not recognised as a distinct genus by the above writers on Trilobitcs. 



The general form and contour of the cephalic shield closely resemble that of Asaplius platycephalus of 

 Stokes (Isotelus 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 Asaplius 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 Asaplius, Phacops, Hemi- 

 crypterus, 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 giyas, 

 but part of a nearly regular circle, as in Hemicrypterus. Like Asaplius, 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 Isotelus, nor yet as narrow 

 as in most species of Asaplius. Like Isotelus, the glabella is but obscurely defined. 



From Dr. Locke's megistos, it difi'ers 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 Iowa 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. S.) 



(Tab. VIII., 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 /. gigas are but very slightly angled, and almost approach 

 an arch. (See Palceontology of New York, PI. 01, fig. 3 c, and /.) 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 Isotelus must be abandoned. 



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