S4 MATTHEW ON OEGANISMS 



there are difFereuces of oruamentatiou and form that show that the species at least are 

 diverse. In Palœaxpis Americana we appear to have a form of shield corresponding to the 

 ventral sente of a Diplaspis, except that the front is more irregnlar and the plan of the 

 striatiou different. Neither in Diplaspis nor in the English genera of Silurian fishes is 

 there an arrangement of the little ridges of the surface of the scute similar to that repre- 

 sented as characterizing the central part of the scute of Palseaspis, in which no corres- 

 pondence in the general course of the ridges to the longer axis of the plate is indicated. 

 The want of symmetry observable in the two sides of the ventral scute in Diplaspis and 

 iu the English Silurian Scaphaspis is also apparent in Palœaspis Americana. Prof. Clay- 

 pole's other species, Palœaspis iruncata is founded, as he himself has observed, on a plate 

 resembling the dorsal scute of Cyathaspis, and the association of the two plates in the 

 same beds would suggest that they may be the dorsal and ventral scutes of the same 

 species. The unique ornamentation of the test, as well as the differences of structure in 

 the plates themselves, would indicate that they are of a different genus from the Acadian 

 and the English forms ; but they occur at nearly the same horizon. 



The genera of Pteraspidiau fishes ' known to the writer are the following : — 



Pal.easi'is, Claypnle. — Scutum simplex, ovale- — Silurian. 



Scaphaspis, Lankefter. — Scutum simplex, ovale — Silurian and Devonian. 



Cyathaspis, lankeskr. — Scutum in quatuor partes divisum, ovale— Silurian. 



Diplaspis (gen. nov.) — Scuta in dorso et pectore in septem (?) partes divisa, ovalia — Silurian. 



Pterasi'is, Nner. — Scutum in septem partes divisum, sagittiforme — Devonian. 



III. — Geological Horizon. 



From variovis considerations it appears that these fish remains are of greater antiquity 

 than was at first supposed. This may be judged from the aspect of the Silurian series iu 

 various parts of Acadia and Gaspé. Near the latter district, in the island of Anticosti, are 

 Silurian limestones, referred by the late Mr. Billings to the Silurian, as follows : — 



Feet. 



Div. 1. — Grey and yellow limestones (Lower Llandovery) 306 



Div. 2. — Grey limestone and dark grey bitumenous limestone (Llandovery) 447 



Div. 3. — Grey and brown sliale; grey and drab argillaceous and bitumenous lime- 

 stones (Llandovery) 540 



Div. 4. — Liglit grey granular limestone (Llandovery) 69 



1,362 



On the mainland of Canada, opposite to Anticosti, is the Gaspé series, which is 

 chiefly Devonian, but has at the base limestones referred to the Lower Helderberg group. 

 No beds of the Niagara period appear in this district, but they are to be found at the head 

 of Bay Chaleurs as a part of the Silurian series. The Lower Helderberg group, there- 

 fore, appears to be connected in this district with the Devonian series. 



^ The diagnoses of the second, third and fifth genera are taken from Prof. Lankester's memoir cited. 

 ^ Quart. Jour. Geol. Soc. London, Feb. 1881. This genus is separated from other Pteraspidian fishes, on 

 account of organic differences in the structure of the plates, and not because of diflTerence of form. 



