no SYSTEMATIC ARRANGEMENT 



b h. — The caudal axis does not project, or projects but very little from the shield. 



8. As. {Isot.) platycephalus : Scuto capitis caudaeque parabolico, acuto ; tlioracis axi lobis latera- 

 libus latiori, axi caudae obsoleta. Long. 2-6". Tab. II, Fig. 13. 



Jief. — Asaph, platyceph. Stokes, Trans, of the Geol. Soc. of London, i, 8. 208, PI. 27. 

 Isoldes gigas, Dekay, Annals of the Lye. of Nat. His. of New York, i, 176, PL 12, 13. 

 Fig. 1. Dalm. Palcead. 70. 13. Green, Mon. of Tril. 67. Bronn, Leth. i, 115, 

 PI. IX, Fig. 8. Emmr. Dissert. 32. 12. Milne Edw. Crmt. iii, 298. 1. Brong- 

 niartia isoleta, Eaton, Geol. Text Book, PI. II, Fig. 19. 



Found in a black limestone of Trenton Falls, in the state of New York ; at Cincinnati, 

 in the state of Ohio, and at other places. My figure, which is perfectly accurate, represents 

 the impression of the lower surface of the shell, and, therefore, exhibits traces of lobes on 

 the glabella and articulations in the tail, which are not recognizable in the upper surface. 

 I convinced myself of the presence of a peculiar sculpture, consisting of dots, from 

 remnants of it in the specimen from which my drawing is taken, which therein indicates 

 a near affinity with Asajjhns expansvs. The acute shape of the posterior cephalic angles and 

 of the lateral lobes of the body distinguishes Isoteles from the group of Asaplms marked B a, 

 even if the shape were not distinct. 



Remarks. — 1. Asajj/iiis angitstifrons, Dalm. {Palcead. 44. 5, Tab. Ill, Fig. 2, a, b), of wbicb I have 

 seen a plaster cast at Berlin, seemed to me to be a member of this group, but the caudal axis projected 

 rather more from the .shield, and was not narrower than the bodj' at the anterior part. This shortening, 

 which seems to be indicated by furrows and little punctations at the lower side of the shell, is a peculiar 

 feature of Asaphus platycephalus. 



2. Green's Isoteles planus [Mon, p. 68), stegops (71), and Isot. megalops are indi\'iduals in different 

 states of preservation, and of diflerent sizes, but all belong to Isot. gigas. The same author's Isot. 

 Cyclops (p. 69), on the other hand, seems to belong to a peculiar species, nearly related to Asapti. angus- 

 tifrons, if not identical with it. Isotel. megalops is based upon individuals which most nearly correspond 

 with the one I have represented. 



D. 



Six-joi?ited TriloMtes, capable of rolling themselves up, and having the axis of the body equally 

 broad throughout. 



Genus 20. — Ampyx, Dalman. 



I am acquainted neither with original specimens, nor with casts of this genus, and 

 cannot, therefore, give a sufficient explanation of it ; judging from the figures and the 

 descriptions of authors, it seems to be most nearly allied to those species of Asaphus which 

 have a pointed cephalic shield. It has much produced angles, which are generally broken 

 off, but it is distinguished from the species alluded to by its higher and more prominent 

 glabella, and more projecting eyes. According to Dalman, the axis consists of six body 

 rings (according to Sars, of five only), which are short, but have broad lateral lobes, on which 

 (according to Sars) the transverse furrows can be seen. The caudal shield resembles the 

 cephalic shield, and has a distinctly projecting, obtuse axis, in which six or eight rings are 

 usually indicated ; the ribs on the sides appear to be wanting. 



