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Plates 1–7.
Glycymeris Da Costa, 1778 (= Pectnculus Lamarck, 1799).
The genus Glycymeris, appearing first in the Cretaceous, is widely spread throughout the Tertiary strata of the world, and exists in the Recent seas of the West Indies, Britain, India, New Zealand, Australia, and western America. It is a shallow-water shell, ranging, according to Woodward, from 8 to 60 fathoms, and rarely to 120 fathoms. Consequently the fossil occurrence is generally in fine conglomerates, sandstones, or shell-beds.
Suter seems to have had an erroneous idea as to which is the anterior of the shell. Thus in his description of both G. laticostata and G. modesta (1913, pp. 851 and 852) he says the beaks curve “forward.” This is not the case. The curvature, though generally slight, is towards the rear. It will not be out of place to quote Fischer (1887, p. 978): “The impression of the anterior adductor is subtrigonal, that of the posterior adductor is subtrapezoidal, and Limited in front by a projection or ridge which is directed towards the umbones and more elevated than the ridge limiting the rear of the anterior adductor; the pallial line at its junction with the impression of the posterior adductor forms a small sinus; the beaks are slightly inclined to the rear.”
Other noteworthy features are—
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(1.) Worm-borings and malformation are very common on the posterior.
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(2.) The ribs are narrower on the posterior.
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(3.) In the huttoni group the anterior adductor impression is larger than the posterior.
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(4.) In oblique shells the axis slopes downwards to the rear.
The earliest known occurrence in the New Zealand area is in the Upper Senonian. Mr. H. Woods (1917, p. 20) has recorded two species from this formation—Pectunculus sp. from the calcareous conglomerate of Amuri Bluff, and P. selwynensis a slightly higher horizon at Selwyn Rapids. Another species, perhaps of Cretaceous age, is Glycymeris concava Marshall, from Wangaloa, a shell very like the Selwyn Rapids one, as noted by Dr. Marshall (1917, p. 459), but distinguishable by its much shallower anal depression. Then, too, the ribs of G. selwynensis are more strongly raised than those of G. concava, a feature seen only in well-preserved specimens. These early forms belong to the huttoni group (see below), differing only in their strongly-raised ribs and wide interstices. In this respect they approach the laticostata group, and may represent the ancestor
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of those shells; but it seems more probable that the separation took place at an earlier stage than that of G. selwynensis.
The Tertiary species fall into three well-defined groups:-
I. The laticostata group, characterized by—
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(a.) Raised rounded ribs (a few exceptions).
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(b.) An equilateral outline, circular to dorso-ventrally oval.
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(c.) Closely set ligamental grooves on a highly inclined area.
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(d.) Valves little inflated, beaks low (exceptions).
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(e.) Teeth small and numerous until invaded by the area.
II. The huttoni group, characterized by—
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(a.) Linear interstices between low, flat ribs (exceptions).
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(b.) An inequilateral outline, subquadrate to obliquely ovate.
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(c.) Well-spaced ligamental grooves on a moderately inclined area.
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(d.) Valves inflated, beaks prominent.
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(e.) Teeth large and strong.
Some specimens of this group present an external resemblance to Cucullaea, but they can always be recognized by their rounded shoulders.
III. The Axinea group, characterized by—
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(a.) Almost smooth surface, under a lens showing very fine radial riblets over the whole disc, superimposed upon a system of narrow obsolete ribs corresponding to the fine crenulations on the inner margin. Sometimes these ribs are shown only on weathering.
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(b.) Outline circular to ovate.
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(c.) Many closely set ligamental grooves on a highly inclined area.
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(d.) Valves inflated, beaks moderate.
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(e.) Teeth numerous and fine.
The fine secondary riblets so well developed in this group often appear in other groups, but then only on a small portion of the disc.
This group corresponds to Axinea of Poli, 1791, used as a separate genus by Cossmarm, Angas, and others; as a section of Glycymeris by Fischer and Adams, and as a synonym of it by Zittel, Dall, and Suter.
The three divisions of the New Zealand species are probably of sub-generic value, the first being Glycymeris s. str., the second a new subgenus, and the third Axinea. The definite classification into subgenera has not been carried out in this paper, as no specimens from abroad were available for comparison.