ROBERTS — DISTRIBUTION OF CEPHALASPIS AND PTERASPIS. 105 



among fish-families can be decided. Probably a kinship existed 

 between the two chief forms of ichthyic test of Cephalaspis and 

 Pteraspis, and it is most likely that our noble friend, the sturgeon 

 (Acipmser), will have to own them of his family; for, as Prof. Hux- 

 ley has lately pointed out, they bear, in shape and arrangement of 

 head-plates, a great affinity to the genns Spatularia, a North 

 American attache of our larger and caviare-giving fish. 



It may help the comprehension of those who are unfamiliar with 

 the osseous head-shields of these old ganoid fishes, if I sketch the 

 two forms whose acquaintance will be most easily made by exploring 

 collectors, Cephalaspis and Pteraspis. Form of shell is a very 

 deceptive guide both in fish and crustacean life ; indeed, if we made 

 our affinities from this alone, one great genus would include many 

 species of both orders, for the shape of Cephalaspidean bucklers is 

 copied almost literally by several Crustacea. A new Harpes from the 

 Silurian limestones of Oesel, figured by Eichwald, agrees not only 

 in shape of head, but even a position of the eyes with Cephalaspis ; 

 while it would be a matter of serious concern where to draw the line 

 between the head-plates of Eurypteris and Cephalaspis. But it is 

 from the closest and most minute examination that species and even 

 families are determined among those which lived during the infancy 

 of vertebi ated life. 



8 



B, Pteraspis Head-shield (as usually found*). 



1, External ridged layer. 



2, Intermediate cancellated layer. 



3, Internal shell layer. 



* A restoration of this is promised us by Prof. Huxley. 



A, Cephalaspis Head-shield. 

 1, Ornamented external layer. 



Pteraspis Luclensis, the oldest fish with which we are at present 

 acquainted, was found in the Lower Ludlow mudstones at Churchill 

 Quarry, near Leintwardine, and has been well described and figured, 

 side by side with its ally, Pteraspis truncatus, from the Upper Ludlow 

 rocks, by Mr. Salter, in the "Annals of Nat. Hist." for July, 1859. 



VOL. IV. 0 



