8 PALEONTOLOGY 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. Cheeks small and narrow ; eyes small. Hypostoma oval, truncated ante- 

 riorly. Segments of body in the adult of the typical species, sixteen ; in the pygi- 

 dium three. 



As stated by Barrande, this genus presents several points of analogy to Para- 

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

 of its thoracic segments, as well 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 work on the Trilobites of Bohemia, if we could do so 

 consistently with the just and inflexible law of priority, which we have endeavored 

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

 individual, would not, we should think, be a sufficient reason for setting it aside, 

 especially as no doubt can be entertained of its generic, and even specific identity 

 with the type of Arionellus. It cannot stand, however, for the reason that it had 

 been used by Oken, in 1815, for a genus of Lepidoptera, and by Lesson for a genus 

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

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

 similarity to Agraulis, used by Boisduval, in 1836, for a genus of Lepidoptera. 

 These names, however, 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 Trigonia and Trigoiia, in Conchology, Cy- 

 prina and Cyprinus, 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 Conocorypfo, instead of Conocephalus or ConocepJialites. The reasons for so 

 doing will be better understood by the following statement of the synonymy of 

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

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

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

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

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

 Ptychoparia and Ctenocephalus 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 was applied to the 

 whole group, he was opposed to retaining either of them, and consequently pro- 

 posed the new name Conocephalites. The law of priority, however, requires us to 

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

 be entertained in regard to the generic identity of its type with the types of the sub- 



