598 BRITISH PALAOZOIC FOSSILS. [ Pisces. 
Microscopic structure: longitudinal medullary tubes numerous, irregular, largest towards the centre, giving off 
numerous branches (or vertical slits) to the circumference, which are simple towards the apex, but give off lateral 
branches lower down; these lateral ramules of the branches terminate in little cells; from all these cavities the 
ealcigerous tubes proceed at right angles; there is a thin outer layer of ganoine, the external furrows of which 
correspond to the internal rows of medullary cavities. 
The genus Dendrodus of Owen has been divided by Agassiz into Dendrodus, Lamnodus and Cricodus, but 
Mr Miller seems to have ascertained that it should be united to Asterolepis: he figures a close row of small 
equal teeth on the edge of the dentary bone, behind which the few great laniary teeth (those formerly described 
as Dendrodus) are far apart, and planted in large, shallow pits on an internal, broad, projecting, horizontal 
ledge. 
DENDRODUS LATUS (Owen). 
Ref—Ag. Poiss. Foss. Vol. IT. t. 55 a, figs. 19, 20; Old Red, t. 28. fig. 1, 2. 
Desc.—Tooth about two inches long, conical, with a slight sigmoidal curve, slightly keeled towards the 
apex, which is obtuse; section circular towards the base, which is nearly two-thirds the length of the tooth in 
width; surface with nearly equal obtuse longitudinal strize, about three in one line at the middle. 
Position and Locality.—Old Red sandstone, Scat Craig. 
8th Fam. PLACODERMATA. 
Body enclosed in a short, oblong case or carapace, composed of large, strong bony plates, either lateral 
in pairs and nearly symmetrical, or odd and placed in the median line on the upper or lower surfaces; spinous 
processes of the vertebree usually ossified, but the bodies not; no distinct caudal nor pectoral fins; teeth 
conical, simple, equal. 
I originally proposed this group (Annals Nat. Hist. July, 1848, p. 6) for all the genera included in the 
Fam. Cephalaspide of Agassiz except his Cephalaspis, to which I proposed to restrict that family name, 
the latter differing from the present fishes in having the body covered with scales of the ordinary Ganoid con- 
struction, distinct heterocercal tail, &c.; but I also suggested that some fragments of other genera, the essential 
characters of which were then unknown, might have the same position, which subsequent researches shew really 
to possess the characters of the Cwlacanthi, to which group they should rather be approximated, leaving the 
definition of this family unchanged. 
Genus. PTERICHTHYS (Ag.) 
Syn. = (Pterichthys Ag. [under side] + Pamphractus Ag. [upper side]) = Homothorax (Ag.) 
Gen. Char.—Body covered by an oblong carapace, flat on the under aspect, vertical on the sides, elevated 
and obtusely keeled along the mid-line of the dorsal aspect, the sides sloping thence to the vertical margins, 
the most elevated point being a littlé in front of the middle; tail thick, conical, covered with rhomboidal 
granulated scales ; one small (? anal or dorsal) fin close to the posterior margin of the carapace, having a small 
thick spine at its front edge, the fin-membrane granulated, without distinct rays; head rounded, of moderate 
size; at its junction with the carapace, two large falciform, pointed, pectoral appendages are articulated 
by a ginglymoid joint, one on each side, exceeding half the carapace in length, thickest about the middle, where 
there seems to be a joint, and having their anterior edge tuberculo-serrate; entire surface of the strong bony 
exo-skeleton covered with coarse granules. 
The carapace, as is clearly proved by Sir P. Egerton and Mr Miller, has invariably the ventral side flattened, 
and the dorsal side keeled or elevated (Agassiz having in some cases described the keeled side as inferior). 
Miller’s observations as to the number of the bony plates in the carapace exactly agree with my own; although 
Agassiz and Sir P. Egerton in their restored figures give a larger number. The whole carapace seems to be, 
as Miller says, composed of eleven thick, bony plates, five on the ventral aspect, i.e. one central rhombic plate 
