g PALAEONTOLOGY OP THE UPPER MISSOURI. 



parallel to the longer axis of the body ; behind the eyes they diverge and extend 

 backwards, so as to cut the straight posterior margin somewhat within the lateral 

 angles. Checks small and narrow ; eyes small. Hj^ostoma oval, truncated ante- 

 riorly. Segments of body in the adult of the tj^pical species, sixteen ; in the pygi- 

 dium three. 



As stated by Barrandc, this genus presents several points of analog)' to Para- 

 doxides, particularly in the arrangement of its facial sutiures, and the great number 

 of its thoracic segments, as avcU as in the small size of its pygidium. In Europe it 

 is only known to have been represented by the single typical species, which is con- 

 fined to the primordial zone. Several species have been described from rocks of 

 the same age in America, and the genus is not certainly known to have existed 

 after the close of the Primordial epoch. 



We would with pleasure adopt the name Arionellus, proposed for this genus by 

 Prof. Barrande, in his splendid Avork on the Trilobites of Bohemia, if we could do so 

 consistently with the just and inflexible laAV of priority, Avhich we have endeavored 

 to obey in all cases. The fact that Corda's first name, Jlerse, Avas applied to a young 

 individual, Avould not, Ave should think, be a sufficient reason for setting it aside, 

 especially as no doubt can be entertained of its generic, and CA^en specific identity 

 Avith the type of Arionellus. It cannot stand, hoAvever, for the reason that it had 

 been used by Okcn, in 1815, for a genus of Lepkloi)tcra, and by Lesson for a genus 

 of Birds in 1837. The next name, then, that Ave are compelled to consider, is 

 Agraulos, Avhich Prof Barrande thinks should be rejected, in consequence of its 

 similarity to Agrcmlis, used by Boisduval, in 1836, for a genus of Lejiidoptera. 

 These names, hoAvcA'er, seem to us to be sufficiently distinct to prevent confusion ; 

 certainly they are as much so as many others retained in various departments of 

 natural history— such, for instance, as Tn'(jonia and Trigona, in Conchology, Cy- 

 prina and Ci/2»'inics, in Conchology and Ichthyology, and Pica and Picus, in 

 Ornithology. 



We should also feel constrained, on the same principle, to adopt Corda's first 

 name Co)iorQryphe, instead of Conocephalus or Conoceplialites. The reasons for so 

 doing Avill be better understood by the foUoAving statement of the synonymy of 

 this genus. In the first place Zenker named it Conocephalus, in 1833. Finding 

 this name had been used, hoAvevcr, in 1812, for a genus of Orthoptera, by Thun- 

 berg, he changed it in the explanations of his figures in the same Avork, to 

 Trigonocepthalus, which had also been previously used by Oppel for a genus of 

 Eeptiles, in 1811. In 1847, Corda applied the three generic names, Conocoryphe, 

 Ptychoparia and Ctenocepthalus to three species of this same genus of Trilobites. 

 In 1852, Prof. Barrande, seeing that none of the older names could be adopted for 

 this group, also rejected Corda's names, on the ground that he had, Avithout suffi- 

 cient reason, divided the genus, and that as no one of his names Avas applied to the 

 Avhole group, he Avas opposed to retaining either of them, and consequently pro- 

 posed the ncAv name Conocephalites. The laAV of priority, however, requires us to 

 adopt the first unoccupied name in this and in all other cases Avhere no doubt can 

 be entertained in regard to the generic identity of its tj^e Avith the types of the sub- 



