28 William Patten. 



Arthropods a group of animals, nearly all of which have osseous, dermal skeletons, and 

 some of "which have such characteristicall}^ vertebrate bodies and fins as the Gephalaspidae 

 sind Asterolepidae; or to include in the vertebrates, animals having many pairs of jointed 

 appendages, it becomes necessary to create for them a new class, one that shall occupy a 

 position between the true vertebrates and Arthropods, and unite these two great groups 

 into one compact phyum. 



I propose, for this new class, the name Peltocephalata, and include in it forms like 

 Pterichthys, Gephalaspis, Pteraspis, Tremataspis, and related genera. The class may be pro- 

 visionally characterized as follows. 



L. The Peltocephalata were arthropod-like animals, moving about through the soft 

 mud on the bottom of shallow waters, in the typical Arthropod position. Most of the body 

 was probably concealed, leaving only the prominent median eyes exposed. The presence of 

 paired oar-like appendages indicates the power of free swimming, but the more or less rigid 

 and clumsy appendages, and heavily armored body, could have produced little more than 

 brief, spasmodic excursions, like those of adult Limuli and Eurypterids, or jerky, intermit- 

 tent flights through the water, like those of a Copepod. And, just as in these examples, the 

 shape of the body and the position of the appendages in reference to the centre of gravity 

 compel the free swimming individual to reverse the usual position of dorsal and ventral sur- 

 faces, so in tlie Peltocephalata the prevalence of the same conditions must have forced them, 

 after leaving the bottom, to turn over and swim with the neural side uppermost, in the true 

 vertebrate position. The swimming movements were aided in some cases by numerous small 

 appendages on the head and trunk, and fishlike caudal fins and tail were also used in swim- 

 ming and in reversing the position of the dorsal and ventral surfaces. 



It was not till this new method of locomotion had completely replaced the old, that 

 the eyes left the haemel surface (their position in most adult Arthropods) and returned to 

 the neural surface of the body, (their position in embryo Arthropods and their permanent 

 position in Vertebrates). 



The Exoskeleton was a true dermal armor of eetodermic origin, intermediate between 

 the type presented by Limulus and that of the more modern Vertebrates. It consisted of 

 three principal layers ; the middle one containing large, more or less regular spaces or can- 

 cellae. The matrix was strongly laminated and penetrated by numerous dentine-like tubul- 

 es, or pore canals, and contained either unipolar or multipolar osseous lacunae. The trunk 

 was covered with rhomboidal scales or with segmentally arranged ring-like plates. The 

 presence of a system of lateral line organs is indicated by numerous, pit-like markings ar- 

 ranged in linear series. 



A flattened cartilagenous cranium was present, but notochord and vertebral arches 

 were absent or rudimentary. Median and lateral eyes were enclosed in bony orbits, some- 

 times protected by hard, convex lens-like coverings continuous with the outer layers of the 



