Ichthyodorulites.—The term ichthyodorulite (ἰχθύς, fish; δόρυ, lance; λίθος, stone) is applied to detached fin-spines, dermal spines, and tubercles belonging to unrecognized species of sharks and chimæras. Some of these are serrated, others entire, some straight, some curved, and some with elaborate armature or sculpture. Some doubtless belong to Cestraciontes, others to Pleuracanthidæ; some to Squalidæ, some to chimæras, and others, perhaps, to forms still altogether unknown.
FOOTNOTES:
[151] Bulletin Geol. Soc. America, 1892.
[CHAPTER XXXII]
THE CLASS OSTRACOPHORI[152]
Ostracophores.—Among the earliest vertebrates actually recognized as fossils belongs the group known as Ostracophori (ὄστρακος, a box; φορέω, to bear). These are most extraordinary creatures, jawless, apparently limbless, and enveloped in most cases anteriorly in a coat of mail. In typical forms the head is very broad, bony, and horseshoe-shaped, attached to a slender body, often scaly, with small fins and ending in a heterocercal tail. What the mouth was like can only be guessed, but no trace of jaws has yet been found in connection with it. The most remarkable distinctive character is found in the absence of jaws and limbs in connection with the bony armature. The latter is, however, sometimes obsolete. The back-bone, as usual in primitive fishes, is developed as a persistent notochord imperfectly segmented. The entire absence of jaw structures, as well as the character of the armature, at once separates them widely from the mailed Arthrodires of a later period. But it is by no means certain that these structures were not represented by soft cartilage, of which no traces have been preserved in the specimens known.
Nature of the Ostracophores.—The Ostracophores are found in the Ordovician or Lower Silurian rocks, in the Upper Silurian, and in the Devonian. After the latter period they disappear. The species are very numerous and varied. Their real affinities have been much disputed. Zittel leaves them with the Ganoids, where Agassiz early placed them, but they show little homology in structure with the true Ganoids. Some have regarded them as aberrant Teleosts, possibly as freakish catfishes. Cope saw in them a huge mailed group of archaic Tunicates, while Patten has soberly and with considerable plausibility urged their affinity[153] to the group of spiders, especially to the horseshoe-crabs (Limulus) and their palæozoic ancestors, the Eurypteridæ and Merostomata.