A new occurrence of Ordovician vertebrates has recently been discovered farther north, in sandstones and shales of the Winnipeg Formation of the Williston Basin. Redrawn and somewhat simplified from the geological maps of the Bighorn Mountain Region published by Darton (1906a, pi.47; 1906b, pi.73). The new astraspid, Pyc7iaspis splendens, new genus, new species, described in this paper comes from the Upper Ordovician deposits of the Bighorn Mountains of Wyomhig.
The histology of the exoskeletonin this form will be treated in more detail in another connection Orvig, in MS., a). According to Rhodes and Wingard (1957, p. 453), Archeognathus primus is in several ways reminiscent of the Neurodontiformes (the "fibrous conodonts" of previous authors). All but two of the specimens figured in this paper (fig. 3) are contained in beds of Upper Ordovician age from the Rock Creek section on the eastern slope of the Bighorn Mountains.
The average size of the plates and scales is somewhat larger than that in the Harding Sandstone of Colorado and the sandstone and shale of the Winnipeg Formation of the Williston Basin in Mon-. The state of preservation of the material is exceptionally favorable for both macroscopic and microscopic examination. Ornamentation of the exoskeleton:a,green from small tubers belonging to an early stage of growth, un- superficial view (mainly to USNM21334, X30); b, large tubers belonging to an overall stage of growth, oblique view (USNM21335, X30).
The polygonal carapace plates most commonly encountered within the material are generally elongated rhomboidal or less distinctly pentagonal in shape.
NEW OSTRAGODERM— 0RVIG H
In most plates and scales, on the other hand, the early shagreen of star-shaped tubercles is, during the later stages of growth, completely replaced by another type of mature ornamentation consisting only of thick tubercles, mushroom-like (fig. 2,6;tf, pi. The neck part of the tubercles, which is always well developed, is slightly compressed immediately below the crown and is separated from the latter by a special nick. to be expected given the state of preservation of the material, the stillness of the crown is of course a primary condition.
In the material of Pycnaspis splendens, a new genus, a new species, that I have available, it is estimated that only 0.4 percent of the specimens are present. It is true that, as far as one can tell, a correspondingly striking and abrupt change in the ornamentation of the exoskeleton with age is not found in any other representative of the Heterostracian, neither in placoderms nor in early teleostomians (cf. e.g. inBothriolepis, Stensio, 1948, pp. In Astraspis desiderata from the Harding Sandstone there is little or no evidence of a similar variation in the shape of the tubercles from one part of the carapace to the other.
The plates and scales of the exoskeleton are all remarkably strong, their average thickness, as in Astraspis and Eriptychius, probably greater in relation to the size of the animals as a whole than in any of the geologically younger members of the Heterostraci. 14 CONTINUATION OF THE NATIONAL MUSEUM vol. loose .. MS., a), is not enamel, nor enamel-like, but clearly a special variety of dentinoiis tissue (mesodentine); and (b) a lower part of typical aspidin forming thick walls around a basal, "pulp". In the tubers of early stages of growth, the upper layer, just as in Astraspis (0rvig, in MS., a), without true dentinal tubules similar to those in true dentine (e.g. in the tubercles and ridges of the exoskeleton in Eriptychius and post-Ordovician Heterostraci in general), however, it is pierced throughout.
On the other hand, in tubercles of successive stages of growth, this layer shows to some extent the dentinous tissue in the exoskeleton of the various early osteostrata (0rvig, in MS., a), since it consists of a thick outer part with the same fine canals as those just mentioned (ezul, fig. 3), and besides this a thin inner part, which contains a system of short, stalk-like dentinal tubes, or, in some cases, elongated cellular spaces, from which these fine canals arise in a superficial direction {bzul, dt". Aspidin in the lower part tubercles is histologically exactly similar to that in the middle and basal layers of the plates and. In the tubers of the following stages of growth, however, a system of very thin, straight vascular canals arises from the radiating manner. the upper part of the "pulp" cavity, which ends near and sometimes almost at the border between the aspidine and external dentine tissue (can.pca, Figure 3).
This system of vascular canals has no connection whatsoever with the short dentinal tubules or cell spaces in the inner part of the tooth tissue. The nature of the hard tissue in the tubercles will be dealt with elsewhere (0rvig, in MS., a) and will not be further commented upon here. Of interest to us in this connection is, above all, the fact that the tubercles in the early stages of growth, due to their microstructure, can invariably be sharply distinguished from the subsequent stages of growth (clearly shown, e.g.
NEW OSTRACODERM— 0RVIG 15 specimens exhibiting simultaneously both the immature and mature
On the other hand, the Pycnaspis-heSirmg beds seem to contain no fragments of bone. To what extent are these beds without other micro-. fossils such as lingulid shells, conodonts, etc., cannot be decided with certainty on the basis of the limited material dealt with here. Plates belonging to Astraspis^.sp. fig. 4) are about the same order of magnitude as those of Pycnaspis splendens, new genus, nov.
In their basal part, there is no large pulp cavity, but a system of vascular canals from which the dentine tubes emerge in a superficial direction (for further details of the microstructure, see Prvig, in MS., a). Only on one of the plates is the position of the living in the cuirass. Interpretation of symbols: bale, basal layer of scales; can.ascb, ascending vascular channels piercing the basal layer; can.asc, vascular canals in the upper part of their middle layer of scales, rising towards the dentine ridge; can.d, short vascular canals that extend outward in the basal part of the dentine ridge and open outward at the basal periphery of these ridges; cati.l, longitudinal vascular canals in the basal part of dentine ridges; dt, dentinal tubules; mil, middle, vascular layer of scales; o.can.tf, openings of vascular canals at the basal perimeter of dentin ridges; sul, surface layers of scales.
These scales, which vary somewhat in size, consist of an unadorned overlapping anterior portion and an exposed posterior portion displaying a system of roughly parallel dentine ridges. As in the scales of Eriptychius from the Harding Sandstone and other formations, the dentine ridges are very reminiscent of those of the exoskeleton in Cyathaspida and Pteraspida (see. In the material from the Pycnaspis hearing beds there are also several scales of large size decorated with tubercles quite thick (pi.. 5, 6) which are to some extent suggestive of median dorsal and ventral median trunk ridge scales in some post-Ordovician Heterostraci, e.g.
As far as can be seen at present, scales of this particular kind are not met with in the Eriptychius material of the Harding Sandstone of Colorado and the Winnipeg Formation of the Williston Basin in. There is certainly reason to believe that the Ordovician vertebrates of North America, as they become better known, will be found to be very high. useful for con-elation pm-poscs, but until now there has been very little information in the literature regarding their stratigraphic distribution.
NEW OSTRACODERM— 0RVIG IQ what has been said in this paper, one may now distinguish two differ-
Fish remains in Ordovician rocks in Bighorn Mountains, Wyoming, with a r^som^ Ordovician geology of the Northwest.
NEW OSTRACODERM— 0RVIG 21
MS., a genus from the lower Upper Devonian of Escuminac Bay in Canada, with remarks on the phylogeny of the dentinal tissues in lower vertebrates.
NEW OSTRACODERM— 0RVIG 23
3-6, USNM 21336, from same stratum and locality as holotype, detached oriented dorsally (3-5) with anterior margin up: 3, lateral view; 4, external view; 5, basal view; 6, posterior view. Explanation of symbols: bf, basal face; 0 (ia, anterior part of outer face, overlapped by ridge scale anteriorly; ogi, lateral faces, overlap of adjacent polygonal plates; ogp, posterior face, overlap of ridge scale posteriorly; ta, tubercles belonging to early stages of growth; tb, tubercles belonging to 1-4, USNM 21342, a disintegrated keel-shaped plate, apparently belonging to a new genus of Eriptychiida, from same depth and same depth as the Pycnaspis specimen shown in Figures 1-3 of Plate 2:1, external view 3; 4, basal view.
5,6,USNM21818, adetachedridge scale, somewhat incomplete, from the same stratum and locality as the holotype of Pycnaspissplendens, new genus, new species: 5, external view; 6, front view.
