But the consensus of opinion favors the belief that the four orders usually included under this head are distinct and at the same time are really related one to another. For our purposes, then, we may regard the Ostracophori as a distinct class of vertebrates. By placing it after the Elasmobranchs we may indicate its probable descent from a primitive shark-like stock.
Fig. 356.—The Horseshoe Crab or King-crab, Limulus polyphemus Linnæus. Supposed by Professor Patten to be an ally of the Ostracophores; usually regarded as related to the Spiders.
On this subject Dr. Dean remarks: "The entire problem of the homology of the dermal plates and 'scales' in the Ostracophores and Arthrognaths is to the writer by no means as clear as previous writers have conceded. From the histological standpoint, admitting the craniote nature of the vasodentine and cancellous layers in the dermal plates, it nevertheless does not follow that they have been derived from the actual conditions of the dermal denticles of the ancestral Gnathostome, as were unquestionably the dermal plates of Teleostomes and Dipnoans. It seems equally if not more probable, on the other hand, that the dermal armoring of the distinct groups may have had an altogether different mode of origin, the product of a crude evolution which aimed to strengthen the skin by a general deposition of calcareous matter throughout its entire thickness. The tuberculation of plates thus acquired might have become an important step in the development of a more superficial type of armoring which is most preferably represented by the dermal denticles of Selachians. Nor, in passing, need the presence of a mucus-canal system in the early plated forms be of greater morphological importance than a foreshadowing of the conditions of Gnathostomes, for this system of organs might serve as well as evidence, in a general way, of relationship with Marsipobranchs. Nor is this evidence the more conclusive when we reflect that no known type of Gnathostome, recent or fossil, possesses open sensory grooves in distinct dermal plates. The presence, furthermore, of a dorsal fin and a 'truly piscine heterocercal tail,' as noted by Traquair, is by no means as Gnathostome-like as these structures at first glimpse appear. For they lack not merely the characteristic radial supports of fishes, but even actinotrichia. Their mode of support, on the other hand, as Smith Woodward points out, is of a more generalized nature, bent scales, homologous with those of the adjacent body region, taking the place of the piscine external supports." The actual position in the system to be finally assigned to the Ostracophores is therefore still uncertain.
Orders of Ostracophores.—Four orders of Ostracophori are now usually recognized, known in the systems of Woodward and Traquair as Heterostraci, Osteostraci, Antiarcha, and Anaspida. The former is the most primitive and perhaps the most nearly allied to the sharks, the second is not very remote from it, the last two aberrant in very different directions. Hay places the Antiarcha with the Arthrodira under the superorder of Placodermi.
Order Heterostraci.—The Heterostraci (ἕτερος], different; ὀστράκος, box) have no bone-corpuscles in the coat of mail. This typically consists of a few pieces above, firmly united and traversed by dermal sense-organs or "lateral lines." The ventral shield is simple. Four families are recognized by Traquair as constituting the Heterostraci, these forming a continuous series from shark-like forms to the carapace-covered Pteraspis. In the most primitive family, the Thelodontidæ,[154] the head and trunk are covered with small scales or tubercles of dentine and not fused into large plates. The tail is slender and heterocercal, the caudal fin deeply forked. Until lately these tubercles were regarded as belonging to sharks, and they are still regarded by Traquair as evidence of the affinity of the Heterostraci with the Acanthodei. Dr. Traquair thinks that a flap or lappet-like projection behind the head may be a pectoral fin. The three known genera are Thelodus, Lanarkia, and Ateleaspis. In Thelodus the scales consist of a base and a crown separated by a constriction or neck. Thelodus scoticus, Thelodus pagei, and Thelodus planus are found in the Silurian rocks of Scotland. Other species, as Thelodus tulensis of Russia, extend to the Upper Devonian.
In Lanarkia the large sharp scales have an expanded base like the mouth of a trumpet. Lanarkia horrida and L. spinulosa are found in the shire of Lanark in Scotland. In Ateleaspis (tesselatus) the skin is covered with small polygonal plates. The lateral flaps or possibly fins take the form of flat rhombic sculptured scales. In this genus the eyes seem to be on the top of the head.
Fig. 357.—Lanarkia spinosa Traquair. Upper Silurian. Family Thelodontidæ. (After Traquair.)