PLACOGANOIDEI.
The last term signifies a form and structure of tail illus trated by fig. 42, and to be seen in the sharks, dog-fishes, and sturgeons of the present day : it results from a prolongation Heterocercal tail (Lepidasteue owns).
of the vertebral column into the upper lobe dn, producing an unsymmetrical form of the caudal fin, which is contrasted with the symmetrical form of the same fin presented by most fishes of the present day, and illustrated by the Leptolepis spratti jormis (fig. 56, p. 144), in which the vertebral column termi nates at the middle of the base of the caudal fin. There are a few exceptional intermediate forms and structures of this fin.
Fain. Placoderinfiia.—The fossil remains of the singular fishes of the extinct order Plaeoganoidei were first discovered about 1813, in formations of the " old red " or Devonian age in Russia, and are preserved in museums at St. Petersburg and Dorpat. The relation of these specimens to the class of fishes was first announced by Professor Asmuss,* and shortly after, the generic names Asterolepis and Bothriolepis were invented by Professor Eichwald,t to express certain modifications of the external surface of portions of the ganoid plates, subse quently recognised as constituting the buckler of the fore-part of the extinct fishes. In September 1840 Mr. Hugh Miller submitted to the geological section of the British Association at Glasgow the first discovered specimens, affording a recog nizable idea of the form of one of these " old red" fishes, and for this form Professor Agassiz assigned the generic name Pterichthys (pteron, a wing, ith,thys, a fish). Although, there fore, the term Asterolepis had been attached to a fragment of the cuirass of this fish a few months previously, yet, as no recog nizable generic characters were associated with such name, and as Asterolepis has been applied also to other genera —e.g., Homosteus and Heterostius of Asmuss—the example of British palaeontologists will be here followed, in retaining the name Pterichthys for the present genus. " Of all the organisms of the system," wrote the lamented Hugh Miller in his work on the Old Red Sandstone, " one of the most extraordinary, and the one in which Lamarck would have most delighted, is the Pterichthys, or winged fish, an ichthyolite which the writer had the pleasure of introducing to the acquaintance of geolo gista nearly three years ago (1840), but which he first laid open to the light about seven years earlier " (1833).
Genus PTERICHTHYS (fig. 43).—The head and the anterior half of the trunk are defended by ganoid plates----ti. e., plates composed of a hard bone coated with enamel ; those of the trunk forming a buckler composed of a back plate (fig. 43) and breast-plate (fig. 44), articulated together at the sides. The rest of the trunk was defended by small ganoid scales, flexible, like scale-armour, and bore a small dorsal fin (fig. 43, d), and a terminal heterocercal fin, very rarely displayed in fossil specimens. The pectoral spines, c, are formed of ganoid material, like the buckler. The armour of the head, or helmet, appears to have been articulated by a movable joint to the trunk-buckler. One of the few existing ganoid fishes (Lepidosteus) is remarkable for the degree in which the head moves upon the trunk. The component dermal plates of the helmet correspond in some measure with the position of the cranial bones in osseous fishes, but not sufficiently to sanction the application to them of corresponding names. They are indicated by figures in the cut 43 : 2 is the front terminal or rostra plate ; it is followed in the median line by four other plates in the following order :-4, premalian ; 6,me dian ; s, postmedian ; io, nuehal ; 3 is the marginal, and 7 the postmarginal ; $ is the prelateral, and 9 the postlateral. The dorsal shield of the trunk-cuirass is composed of two mid plates and two on each side. is is the "dorsomedian," the poskl,orsomedian ; is the dorsolateral, 13 the post-dorso lateral. The ventral shield (fig. 44) consists of one mid-plate and two side-plates : :3 is probably a part of the cephalic shield or of the mandible : 19 is the ventrolateral, 21 the postventrolateral ; the small supplementary plate marked i7 is usually confluent with :9 ; 1[6 is the ventromedian plate ; its margins are bevelled off and overlapped by the lateral plates.