so MATTHEW ON OEGANISMS 



aud in this respect corresponds to the remarkable armoured fishes (Placoganoids) of the 

 Devonian age. The general form and size of these plates will be seen by reference to 

 fig. 1 of the accompanying plate. 



The plates on the left (a, b, V and c) represent the main part of the dorsal covering ; 

 those on the right {d especially) the covering of the ventral surface of the head and tho- 

 racic parts of the fish. Plate a is the rostrum ; h aud V are the lateral cornua ; c the 

 dorsal scute. The large plate [d) on the right was attached to the ventral surface, and 

 may be called the ventral scute ; e and e' are fragments of lateral plates which probably 

 find their place behind the lateral cornua, as posterior lateral plates. There are two 

 broken fragments of plates (/ and g) in front of the rostral plate, whose place is not 

 known. 



Only two of these plates are complete — the rostrum and one of the lateral cornua — 

 but the general form of the others may be inferred from the parts preserved, from the 

 corresponding plates of other species and from the markings or sculpture of the parts of 

 the plates that have been preserved. The analogy of these plates to the corresponding 

 plates of known pteraspidian fishes cannot be doubted. 



The plates were found spread out on a layer of shale in the attitude given in figure 

 No. 1, which exhibits the interior view of all the principal plates ; as though, after the 

 death of the animal, the bucklars had been burst apart at one side, but still held by the 

 cartilage at the other. 



The most obvious character by which these Silurian fish plates can be recognized is 

 a fine but very distinct ridging of the surface, the details of the ornamentation being 

 governed by the position of the plates in the series of plates with which the body was 

 covered. 



The rostral plate, «, is marked by little ridges arranged parallel to each other and to 

 the longer diameter of the plate, but ti'ansverse to the axis of the body of the fish. 

 Toward the lower edge of the rostrum the ridges are less regular and anastomose, and at 

 the lateral ends of the rostral plate these ridgelets are connected by iiTcgularly placed 

 cross-ridges, which give the surface of the plate in this part a reticulated appearance. 

 The two anterior lateral plates or cornua, h and b\ also have parallel ridgelets that are 

 parallel to the longer diameter. 



In the dorsal scute, c, of which the anterior part is preserved in the specimen figured, 

 the arrangement of the ridgelets which cover the surface is more complex. Near the an- 

 terior is a small tubercle, with a depressed or perforated centre (see fig. 1) enclosed within 

 two ridges, which form an obovate border to it (see fig. 2 — this part of the plate enlarged), 

 the whole being somewhat elevated above the rest of the plate. This little tubercular 

 elevation stood in an iri'egular oval space (see fig. 2), in which the ridges are more or 

 less concentric to the elevation ; at each side the border of this space runs out somewhat, 

 marking the point where two grand sets of ridgelets meet, one from the anterior, and the 

 other from the posterior end of the plate. Those in front of the oval space form a trian- 

 gular area in which the ridges on the plate have their ends directed toward the tubercle, 

 except at the front of the triangle, where they become transverse to the longer axis of the 

 dorsal scute ; all the ridges in this triangular area are irregular in their course, and fre- 

 quently double upon themselves, or anastomose. There is no suture at the junction of 

 the oval area with the triangular area, or the main part of the dorsal scute behind it, but 



