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Lucinidae.
Recently A. Chavan (1937, 1938) has published a valuable essay on the classification of the Lucinidae. Although the writer does not accept all the conclusions concerning New Zealand groups, he would like here to record his appreciation of M. Chavan's work. Many more of such family revisions are needed by workers in Mollusca.
Gonimyrtea Marwick, 1929.
Chavan (1938, p. 230) has stated that Gonimyrtea (inadvertently attributed to Finlay) should be synonymized with Lucinoma Dall. The type of Lucinoma, the East American Lucina filosa Stimpson, is a large shell, some 35 mm. in diameter, with strong, distant, concentric lamellae and high, alate, dorsal, posterior and anterior areas. The lunule is not deeply sunk, and the cardinal teeth are well separated from the lunular and ligamental margins. On the other hand, the shells of Gonimyrtea are small, under 10 mm., and the sculpture consists of fine, close, concentric ridges. Thus, the two groups are easily separated on external appearance alone. The hinge of Gonimyrtea is more crowded than that of Lucinoma, and the laterals are better developed. The interior of Gonimyrtea is generally strongly striate, and the margins are sometimes weakly crenate. The three species, G. concinna Recent and Pliocene, G. discus Miocene and G. bucculenta Oligocene, form a lineage that is clearly defined and so should be systematically recognised. To the writer, it seems that the lineage is no closer, if indeed as close, to Lucinoma than it is to Loripes (= Lucinida) where Hutton first put it. True Lucinoma is, however, represented in New Zealand by the Miocene L. taylori Powell, 1935.
While discussing Lucinoma it is advisable to consider the following statement by Chavan (1938, p. 82): “‘Miltha’ ampla Hutton from the Wanganui beds in New Zealand is, according to an example in the Cossmann collection, also a Lucinoma.” Some mistake has arisen here, perhaps a wrong identification originally. The species concerned is presumably Mysia ampla, the only use in mollusca of this specific name by Hutton. Where the combination with Miltha came from is hard to say. It has not been suggested in any publication known to the writer. Hutton's original generic placing, Mysia Leach, 1827 = Diplodonta Bronn, 1831 = Taras Risso, 1826, is substantially correct.
Pteromyrtea Finlay, 1926.
Another New Zealand genus discussed by Chavan (1938, p. 231) is Pteromyrtea. This he suggested to be a branch of Cavilucina Fischer, type, the Eocene L. sulcata Lamk, influenced, no doubt, by the obsolescence of the cardinal teeth. Now, there is a widespread tendency in the family to obsolescence of the teeth, so that parallels are very likely to develop, and must be taken into account in classification. Consequently it seems to the writer that a number of very different lineages are combined in Chavan's conception of the “phylum” Cavilucina.
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In its young stages, at least up to 10 mm. diameter, P. dispar, the genotype of Pteromyrtea, has well developed cardinals and laterals and agrees closely in hinge and general shape with the adult of the Miocene P. laminata Hutt. This adult shows no signs of obsolescence of the cardinals, but is undoubtedly an ancestor of dispar. In general appearance, neglecting the obsolete hinge, Cavilucina sulcata shows more resemblance to Gonimyrtea than to Pteromyrtea, which is strongly alate, both anteriorly and posteriorly.
Incidentally, Chavan (1938, p. 88) cites “Loripes” laminatus Hutton as a Myrtea. He was probably misled by specimens in the Cossmann collection, wrongly labelled so, and really the new species described below as M. valdesculpta. Suter confused this common Awamoan shell with Pteromyrtea laminata (Hutt.) which is known to the writer only from White Rock River, Pareora.
Myrtea, Eulopia, Notomyrtea.
There has been a good deal of confusion, for much of which the writer must plead guilty, in the use of Myrtea, Eulopia and Notomyrtea for a number of New Zealand fossils. The type of Myrtea Turton, 1822, is by monotypy the Recent Mediterranean Venus spinifera Montagu; that of Eulopia Dall, 1901, the Recent Lucina sagrnata Dall, from the Gulf of Mexico; and that of Notomyrtea Iredale, 1924, the Recent, New South Wales Myrtea botanica Hedley. Dall separated Eulopia because of the interstitial radials, but Iredale gave no justification for Notomyrtea. Examination of more material since the writer (Marwick, 1928, p. 911) stressed the systematic value of the radials compels him to reverse his opinion and to agree with Cossmann and Chavan that they are often quite variable. Nor has any other criterion been found by which to recognize any distinct lineages within Myrtea, and until such criterion is produced, Myrtea should be used for this very compact and easily identified group. True Myrtea is stated by Chavan to be known first from the Oligocene of the United States, then from the Miocene of Europe. The hinge of the Danian Eulopia microlirata Fin. and Marw. (1937) is unknown, so the generic position is obscure. An undoubted Myrtea, however, M. staminifera Marw. occurs in the Duntroonian (Oligocene) at Chatton and Wharekuri.
Gonimyrtea discus n. sp. Plate 26, fig. 13; plate 27, fig. 26.
Shell like the Recent concinna Hutton but having a less prominent umbo and shallower lunule, also a higher dorsal margin, giving an almost circular outline. The lunular margin of the right valve in concinna is strongly convex, encroaching on the left lunule, but in discus the margin is almost straight. The right anterior lateral tooth of concinna is sometimes very weak, but in discus it is absent altogether. There is a weak right posterior lateral. The irregular internal radial ridges that tend to crenulate the margin of concinna are finer in discus and the margin is smooth.
Height, 6.8 mm.; length, 7 mm.; inflation (1 valve), 1.7 mm. Locality: G.S. 951, Target Gully (Awamoan).
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Pteromyrtea exilis n. sp. Plate 26; figs. 21, 23; plate 27, fig. 27.
Shell small, thin, subcircular, moderately inflated. Anteriorly and posteriorly alate. Lunule deeply excavated, extending across the hinge plate. No escutcheon. Sculpture of fine, regular, concentric lamellae, 6 per mm., with wider interspaces which bear weak radial threads. Lamellae drawn up to crest along posterior dorsal margin, also stronger on anterior area. Right hinge having only posterior cardinal and weak anterior lateral; left hinge having anterior cardinal almost wholly confluent with lunular margin, posterior cardinal present, and extremely weak posterior lateral.
Height, 6.5 mm.; length, 6.5 mm.; inflation (1 valve), 1.5 mm.
Localities: G.S. 2110 (type) and 2222, siltstone Allday Bay; 1160 Awamoa Creek; (Awamoan).
The holotype has weaker sculpture and is more circular in outline than the specimens from other localities. The single right valve from Awamoa Creek (Plate 26; fig. 20) has two well-developed divergent cardinal teeth, and so seems to have developed a left hinge.
Myrtea valdesculpta n. sp. Plate 26; figs. 16, 22; plate 27, fig. 29.
Shell small. sub-triangular, stout. Sculpture of distant, raised lamellae 3 to 4 per mm., the flat interspaces of many specimens bearing wavy radials 8–12 per mm.
Height, 7 mm.; length, 7.8 mm.; inflation (1 valve), 1.6 mm.
Localities: G.S. 951, Target Gully (type); G.S. 1912, Pukeuri; G.S. 2218, Rifle Butts; G.S. 2222, Siltstone, Allday Bay; (Awamoan stage).
This is the common Myrtea of Awamoan horizons, distinguished from the Chatton staminifera (Duntroonian) by the more distant lamellae and more trigonal shape. It was generally mistaken by Suter for Pteromyrtea laminata Hutton, but Finlay (1926, p. 461) long ago pointed out that the two were different. His statement, however, that Suter's figure represents, not laminata, but this hitherto undescribed species cannot be accepted. There seems no reason to doubt the caption to Suter's figures (1915, p. 64), which states that they represent the holotype. A gerontic specimen from G.S. 1160 Awamoa Creek measures 10 mm. × 12 mm.
The radials are not always present.