about 4 (3 to 6) parallel, longitudinal grooves. Hinge-area in left valve with a prominent bulge just in front of the beak, merging into a deep anterior sinus in the hinge-margin.

The anterior byssal ear of the right valve should not be confused with the anterior wing which, like the posterior wing, is the flattened extension of the disc bordering the dorsal margin.

The New Zealand species, P. richmondiana, has been chosen as type instead of the older ochotica Keyserling because duplicates will be more readily available and the possibility seems greater of obtaining specimens that will show further details of the hinge and musculature.

Owing to the very thin shell, specimens showing the hinge-details clearly are extremely rare; and, although a considerable amount of material was examined during the present survey, no example was found showing any trace of a muscle scar, nor was a complete hinge of either valve seen. Even casts of the hinges were difficult to get, largely owing to the hinge-area being so narrow that the break in the enclosing rock, necessary to reveal the imprint, generally runs irregularly along it and destroys it. Also the matrix of many of the Entomonotis beds is too coarse to preserve the finer hinge details. However, sufficient examples were obtained to show the essential hinge-features, though further material may modify the present results to a small extent.

Wilckens (1927, p. 12, pl. 1, fig. 23c) has figured and described the hinge as possessing “two narrow teeth, directed posteriorly at a sharp angle,” but what he took for teeth is the imprint of the right anterior ear on the internal cast owing to the right valve having slipped down slightly from its normal articulation with the left. The hinge is undoubtedly edentulous, as is to be expected from the systematic position of the genus.

King's figures of P. speluncaria (1850, pl. 13) show the right valve with a short, straight hinge-line and an anterior ear and notch rather like that of a Chlamys, that is, the ear is in the same plane as the disc and is not overlapped by it. The byssal ear of Entomonotis, on the other hand, is set obliquely to the disc and is somewhat overlapped by the anterior wing, which, moreover, is turned down to form a submargin parallel to the ear.

Waagen (1881, pl. 22, f. 3c) has published a figure showing the left hinge of P. kazanensis, from the Permian of the Salt Range. This hinge has an altogether different shape anteriorly from that of Entomonotis, thus furnishing another reason in favour of generic separation.

Occasional specimens, both right and left valves, on which the surface markings are well preserved bear regular, well spaced, concentric ridges which are stronger on the radial ribs than in the interspaces.

The small, curved, transverse ridges on the lower anterior surface of the byssal ear probably functioned as a ctenolium. Prints of these ridges (Fig. 4) are to be seen in most specimens that show the byssal ear or its cast; but owing to their concealed position they have generally escaped attention. The only reference to them found was that of Teller, whose fig., 9, pl. 18, shows the ridges cearly and

bears the legend “mit gestreiftem Byssohr.” One specimen from Mount Heslington (Fig. 33) which still has some of the shelly surface remaining also shows a ctenolium on the lower side of the notch, the teeth being apparently raised continuations of growth striae. The earlier of these ridges are transverse, but the later ones run obliquely along the notch, and are thus almost at right angles to the ridges on the ear.

Entomonotis richmondiana (Zittel). Figs. 4, 9, 13–24, 33.

1864. Monotis salinaria var. richmondiana Zitt., Novara Expd. Geol., Th. 1 bd., 2 abt., p. 26, pl. 6, fs. 1, a-e.

Entomonotis generally occurs in great profusion, at many places forming bands 10ft. to 20ft. thick, crowded with casts of the valves. These present a bewildering range of form and sculpture, so that although systematists have tried to separate them into carefully described varieties, the results are not satisfactory. Thus, when Keyserling first introduced his Avicula ochotica he described three varieties, major, media, and minor. Teller (1886, p. 116) considered these as not tenable, but he proposed the new varieties densistriata, sparsicostata, eurhachis, ambigua, and pachypleura, remarking that Keyserling's var. major was very close to his own densistriata and pachypleura. According to Trechmann, Frech, whose work is not available in New Zealand, considered “that the three circum-Pacific species… ochotica, richmondiana, and subcircularis are varieties of one and the same shell. He believes that P. richmondiana occurs in two forms, and figures and describes a new form as var. truncata.”

Trechmann (1918, pp. 191–196) recognised P. ochotica as occurring in New Zealand represented by the five varieties of Teller, and in addition he described the new variety acutecostata. He, however, considered richmondiana as a separate species occurring at a slightly lower horizon in the Noric than ochotica. Further, Trechmann identified as Monotis salinaria Bronn, in a limestone from Okuku, Canterbury, shells which are undoubtedly Entomonotis. Several right valves have now been excavated, and all show the characteristic byssal notch. Monotis must therefore be deleted from New Zealand lists and the varieties intermedia and hemispherica Trechmann (p. 196) must be added to the list of richmondiana varieties. The variety hemispherica, for which the original of Trechmann's pl. 20, fig. 3 is hereby designated lectotype, has the broad, smooth, simple ribbing of pachypleura, but is small, and more inflated than Teller's figured types. Variety intermedia, for which the original of Trechmann's pl. 20, fig. 1 is hereby designated lectotype, closely agrees with the typical richmondiana.

It is difficult to decide just how to name the New Zealand examples of Entomonotis. Recent collections by officers of the Geological Survey from Mokau, Kawhia, and Eighty-eight Valley do not confirm Trechmann's idea that two distinct species, ochotica and richmondiana can be distinguished at different horizons, but in the absence of actual specimens from Siberia, it is perhaps hazardous to form a decided opinion as to the relationship of the New Zealand species to ochotica. Judged by Teller's figures, the two groups are doubtfully separable specifically; but the geographic facts should probably be taken as a guide. Accordingly richmondiana is here