HISTORICAL REVIEW. 343 



Hugh Miller, the discoverer of Pterichthys, says (Old Red Sandstone, p. 50), in 

 comparing a trilobite with Cephalaspis, " The fish and the crustacean are won- 

 derfully alike." "They exhibit the points at which the plated fish is linked to 

 the shelled crustacean." Agassiz was at first in doubt as to whether Pterichthys 

 was a fish or a crustacean. 



Sir Roderick Murchison, when first shown specimens of Pterichthys wrote 

 regarding them that, "If not fishes, they more clearly approach to crustaceans 

 than to any other class." Again, "They (Cephalaspis and Pterichthys) form the 

 connecting links between crustaceans and fishes." 



InSiluria (London, 1854, p. 252), speaking of Cephalaspis agassizii, he says: 

 "This fish with its large buckler-shaped head and its thin body, jointed somewhat 

 like a lobster, is perhaps the most remarkable example of a fish of apparently so 

 intermediate a character that the detached portions of its head when first found 

 were supposed to belong to Crustacea." In a footnote Murchison adds: "Mr. 

 Miller has requested his readers to compare the head of Asaphus (now Phacops) 

 caudatus, a well-known silurian trilobite, with that of C. lyellii, to illustrate how 

 the two orders of crustaceans and fishes seem here to meet in the view of persons 

 who have not mastered the subject." 



Eichwald says (1854, page 105): "It is very remarkable that this colossal 

 crab (Pterygotus) formerly regarded by L. Agassiz as a fish, occurs in the 

 dolomitic chalk of Rootzikiill in Oesel, together with another genus, Thyestes, 

 standing between crabs and fishes and resembling Bunodes and Cephalaspis." 



The genus Pteraspis was first proposed by Rudolph Kner, in 1847, to include 

 the forms described in 1835 by Agassiz as Cephalaspis lewisii, and C. lloydii. 

 Their appearance was so unlike the ordinary fish remains that for a long time 

 Kner did not suspect that they had been already described by Agassiz in his 

 Poissons Fossils. From a study of their minute structure Kner believed them to 

 be the internal shells of cephalopods allied to Sepia. 



In 1856, F. Roemer described a form closely related to C. lloydii as Palaeoteu- 

 this, and referred it to the sepiidae, but suggested that the forms described by 

 Kner were crustaceans related to Dithyrocaris or Pterygotus. 



In 1855, R. W. Banks in his paper on the Downton Sandstones, after com- 

 menting on the association in these beds of Lingula cornea, Pterygotus and Pter- 

 aspis (Cyathaspis), made the following observation, page 98, "On the underside 

 of the sharp projections before referred to (on either side of the rounded snout) 

 are protuberances which seem to be projecting horny eyes similar to those of 

 crustaceans." 



He remarks further on, that doubtful as it is whether the buckler-like fossil 

 remains above referred to belong to fishes or to crustaceans, it is certain that they 

 are closely allied to Cephalaspis lloydii and C. lewisii. In a final note, it is an- 

 nounced that Professor Huxley is now minutely examining their structure to 

 determine their true relationship either to the crustaceans or to the fishes. When 

 Huxley's paper appeared, although he gave a very good description of the minute 



