388 THE AMERICAN NATURALIST.  [Vor. XXXVI. 
The Resemblance between the Tremataspide and the Arthro- 
pods.—The following summary of the principal characteristics 
of the Tremataspidz shows how strongly they resemble the 
arthropods, and how surprisingly the resemblance has been 
strengthened at certain points by important discoveries in 
unexpected directions. 
The Tremataspidz resemble the arthropods in (a) their gen- 
eral arthropod appearance, (4) in the minute structure of the 
shell, (c) in the structure and arrangement of the openings for 
the eyes and olfactory organs, (7) in the character and arrange- 
ment of the oral plates, (e) in the absence of vertebrate jaws 
and mouth and in the presence of a small centrally placed oral 
opening similar to that of an arachnid, (/) in the presence of 
numerous pairs of jointed appendages. 
The presence of segmented appendages in Tremataspis and 
related forms is indicated by the following evidence: (1) the 
presence of a pair of oarlike jointed appendages, unlike those 
of any true vertebrate, in Pterichthys, Bothriolepis, Cyathaspis, 
Pteraspis, Tolypaspis, and Tremataspis; (2) the presence of a 
fringe of jointed and movable appendages (25-30 pairs) along 
the ventral margin of thetrunk of Cephalaspis ; (3) the presence 
of a pair of crushing mandibles like those of an arthropod in 
the head of Cephalaspis; (4) the presence in Pterichthys, 
Bothriolepis, and Tremataspis of oral plates that appear to be 
movable laterally, like the jaws of an arthropod ; (5) the presence 
in Tremataspis of a series of eight other pairs of openings like 
the ones to which the large swimming appendages are attached ; 
(6) the presence in Tremataspis of two pairs of entapophyses 
which, like those in Limulus, apparently serve for the attach- 
ment of muscles moving several pairs of appendages; (7) the 
presence in Cyathaspis and Pteraspis of from four to six radiat- 
ing grooves on the inner surface of the dorsal shield, which, like 
similar markings in Limulus, indicate the points of attachment 
of dorso-ventral muscles moving several pairs of appendages. 
The concurrent testimony, from so many different and 
independent sources, to the fundamental similarity between 
the Tremataspidz and the arthropods shows very clearly that 
the resemblance between them is due, neither to incidental 


