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Genotype: Trachycardium (Ovicardium) rossi n.sp.
This shell belongs broadly to the Trachycardium group, fairly large, high, oval shells with strong ribs. The following groups fall into this category, and their use as subgenera under Trachycardium recognises their differences yet preserves what appears to be an obvious relationship:—
Trachycardium Moerch, 1853; Phlogocardia Stewart, 1930; Mexicardia Stewart, 1930; Dallocardia Stewart, 1930; and Regozara Iredale, 1936.
Agnocardia Stewart was also proposed as a subgenus of Trachycardium in 1930, but it does not have the oval form characteristic of the groups mentioned, and may not be closely related. Regozara is an Australian group, the others all belong to the tropical and sub-tropical seas of America, some eastern, some western. Ovicardium might reasonably then, be expected to show closer agreement with Regozara than with the American subgenera, but such is by no means obvious. About the only common point of difference from Trachycardium s.str. shown by both Austral groups is the larger lateral A II, and even here there is much difference in shape, the general disposition of that of Regozara being more like that of Trachycardium. Apart from this small hinge difference, Regozara differs from Trachycardium only in details of sculpture. Ovicardium, however, differs from both and also from the other groups in several important features as well as sculpture—namely, (1) the oblique groove across the left hinge; (2) the long, deep groove above A II; (3) the concave lunule. Although we do not yet have a record of it, Ovicardium probably has existed as a separate entity since well before the Pliocene.
Trachycardium (Ovicardium) rossi n.sp. (Pl. 35, Figs. 7, 8.)
Sculpture of 51 smooth, strong, radial ribs, in general nearly rectangular in cross-section, with interstices of about equal width. Anteriorly, for about the first 5, the ribs are more rounded, closer and crossed by low, regular spaced ridges. Posterior area not strictly
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defined, the more posterior ribs gradually becoming more rounded and lower. The most posterior three much broader.
Height, 102 mm.; length, 95 mm.; inflation (1 valve), 35 mm.
Localities: G.S. 1541, Te Reinga Falls, Hangarua River (type) (Waitotaran); G.S. 1244, Mangatuna Quarry, Uawa Surv. Dist.; Sandstone, Tokomaru Bay. (Opoitian.)
This species is named in honour of Captain D. H. K. Ross, formerly of the Geological Survey, to whom I owe the fine photographs of this fossil.
Trachycardium (Ovicardium) parki n.sp. (Pl. 36, Figs. 19, 22, 23.)
Shell large, high oval, well inflated. Sculpture of 40 squarish main ribs with interstices about half as wide, the rib bounding posterior area of double width; on posterior area are 8 somewhat rounded ribs with shallow interstices, the most posterior two ribs wider than others. Hinge worn but apparently without oblique groove, anterior lateral relatively small.
Height (if complete), 95 mm.; length, 90 mm.; inflation, 35 mm.
Locality: G.S. 634, shell beds, mouth of Butler's (Ototoka) Creek, Nukumaru Surv. Dist. (basal Castlecliffian).
This species is named in honour of Professor James Park, who collected it in 1886. It was listed (Park, 1887, p. 53) as Cardium multiradiatum Hutton (rightly of Sowby.).
The single specimen is fragmentary and badly worn, as are many of the shells from this locality. Indeed, some of them have been derived from the erosion of older beds, so that T. parki may be older than Castlecliffian.
It is easily distinguished from T. rossi by the double rib bounding the posterior area, by the narrower rib-interspaces, by the more narrowly rounded umbo yet more inflated disc, and by the considerably smaller anterior lateral tooth, and lack of a deep socket above it.