Description.—Type specimen representing the larger part of the cranium with supra-occipital and lower jaws, the anterior vomerine portion wanting.
The cranium measures about 18 cm. in height; 17 cm. in length; and about 12 cm. in width.
The cranial region, on the left side, shows the bones of the frontal region much crushed, with the pre-frontal and ethmoid truncated by fracture.
The ossified sclerotic plates are well preserved; the bony ring has a diameter of 43 mm., with the plates measuring 15 mm. in depth.
Maxilla, in widest part, measuring 32 mm. Total length of maxilla as preserved, 13.5 cm. Dentary series with backwardly curved and conical teeth, varying from 5 to 9 mm. in length, and plicate near the base. Sixteen of these teeth are preserved, but the entire series for the ramus probably numbered about 36. Premaxilla wanting. Mandibular ramus strong and deep, at the widest part 4.3 cm.
Right side of the cranium shows the frontal bones much crushed, the sclerotic plates present, but obliquely displaced. Maxillary with only six teeth preserved. Mandibular ramus 4.7 cm. deep.
Matrix of Specimen.—The rock in which the cranium of the fish was embedded is a hard, marly greensand, of a greenish grey colour. Washings of the rock yield a residue which is almost purely glauconitic. Amongst the glauconite casts of foraminifera were recognised, infillings of Globigerina, ? Rotalia and a plane-convex rotaline form which may be Globotruncana (a Cretaceous genus).
Observations.—The present species, Portheus dunedinensis, was, when complete, rather more than half the length of Portheus molossus Cope (1875, pp. 184, 194, fig. 8 woodcut, pls. xxxix-xli; pl. xliv, figs. 5, 10, 11; pl. xlv, figs. 9–11). In the latter the cranium is higher and the teeth of the maxilla are not curved at the apex as in P. dunedinensis.
In its general dimensions P. dunedinensis compares very closely with Portheus australis (A. S. Woodward, 1894, pp. 44, pl. ix, figs. 1, 1a) from Clutha Station, near Hughenden, Queensland. These Cretaceous beds, with an accompanying fauna of reptilian remains, are now referred to the Tambo Series, equivalent to the Upper Albian of Europe. The teeth in the New Zealand species, however, are stouter and broader at the base, and are less regularly spaced on the maxilla, as compared with the Queensland P. australis, in which they are fairly evenly arranged.
E. T. Newton (1877, p. 511, pl. xxii, fig. 13) has described Portheus daviesii, from the Lower Chalk (Turonian) near Maidstone, Kent, but here the teeth of the maxilla are in a shorter series, and are more slenderly conical than in P. dunedinensis.
The same author also described another species of the genus as Portheus gaultinus (Newton, 1877, p. 512, pl. xxii, figs. 1–12 and woodcut), from the Albian (Gault) of Folkestone, Kent. This form differs from the New Zealand species in the much smaller size and lighter build of the maxillary, and in the generally longer and slenderer teeth.
Portheus lewesiensis (Agassiz), which was described by that author under the genus Hypsodon (1843, p. 99, pls. xxv a, xxv b), has a very heavy type of mandible, and the teeth are correspondingly strong, short, conical, and broad at the base.
Fig. 3.—Eothyrsites holosquamatus. gen. et sp. nov. Side of cranium with part of mandible and teeth. Mail-pit. Burnside. Green Island, near Dunedin, N.Z. Upper Oligocene Syntype. Two-thirds nat. size.
Fig. 4.—Eothyrsites holosquamatus, gen. et sp. nov. Pectoral fin. Marl-pit. Burnside, Green Island, near Duuedin. N Z Syntype Two-thirds nat. size.
Fig. 5.—Eothyrsites holosquamatus, gen. et sp. nov Same as Fig. 2. showing strong character of squamation, with veitebiae. Enlaiged one and a-half times.
Locality and Horizon.—The above-described fossil was found in the glauconitic mudstone at Abbotsford, near Dunedin. It occurs about 500 feet above the horizon which contains a Wangaloan fauna, referred to by New Zealand stratigraphists as either Palae-eocene or Danian.
From the previous records of the genus Portheus it is a distinctly Cretaceous genus. The Abbotsford bed, therefore, can hardly be regarded as younger than Danian.