Dacrydium Torell, 1859

Oliver, P. Graham & Holmes, Anna, 2004, Cryptic bivalves with descriptions of new species from the Rodrigues lagoon, Journal of Natural History 38 (23), pp. 3175-3227 : 3188-3189

Genus Dacrydium Torell, 1859

Type species: Mytilus vitrea Holböll in Möller, 1842.

Dacrydium cf. zebra Hayami and Kase, 1993

(figures 50–52)

Dacrydium sp. Kase and Hayami, 1992: 58, 448.

Dacrydium zebra Hayami and Kase, 1993: 46 , figures 148–158 .

Material examined. One lv and 2 rv from Grand Pate´, off Port Mathurin, shell gravel, 17 m. Figured valve, NMW . Z.2001.061.00017.

Measurements (mm, L 6 H). 1.21 6 1.89, 1.29 6 1.90, 1.36 6 2.16.

Description. To 2.1 mm in length. Outline rounded, modioliform with arcuate dorsal margin becoming rounded one-third of the distance from umbo. Rounded anterior margin, straight ventral margin, becoming rounded in last third of shell. Hinge (figure 51) bears about 27 prosoclinal denticles to the posterior of the umbo and a few tiny denticles are present just below the umbo. External sculpture has many evenly spaced, fine concentric lines, shell is white translucent with white opaque blotches.

Prodissoconch is 210 M m (figure 52) and pitted with craters that appear in a concentric pattern, increasing in size toward the outer edge.

Comparisons. The genus Dacrydium contains primarily deep-water species that lack sculpture and colour pattern, and are separated on outline. Variation in outline has been recorded by Allen (1998) and most species have been described from cold water. Despite the few valves found at Rodrigues we have excluded those species described from the Indian Ocean by Poutiers (1989) because they do not conform in outline or pattern. In outline our specimens are closest to D. nipponicum (Okutani, 1975) (although it has a more angular anterior margin and continuously round ventral margin) and quite different from D. occidentale ( Smith, 1885) (almost rectangular in outline apart from the protruding anterior margin) which is common in the western Indian Ocean.

Dacrydium zebra Hayami and Kase, 1993 is in all respects similar to the Rodrigues shells with only a slight difference in that the posterior margin slopes a little more sharply and the anterior is a little more gently rounded. Given the small quantity of material available and possible variation in outline we conclude that the Japanese and Rodrigues shells are conspecific.

Habitat. In Rodrigues the few valves found were in shell gravel at base of reefs at 18 m.

Figures

FIGS. 148–151. Glossocardia obesa, NMW.Z.2001.061.00043: (148) external left valve (MontageTM); (149) external right valve (MontageTM); (150) external right valve (SEM); (151) hinge view, left valve (SEM).

FIGS. 152–157. Thracia arienatoma n. sp., NMW.Z.2000.086.00015: (152) external left valve (MontageTM); (153) internal left valve (SEM); (154, 155) hinge views (SEM); (156) lithodesma (SEM); (157) prodissoconch (SEM).

FIGS. 158–163. (158–160) Thracia arientoma n. sp., NMW.Z.2000.086.00015 (SEM): (158) external right valve; (159, 160) sculpture. (161–163) Thracia kowiensis, NMW.Z.1955.158.815 (SEM): (161) external right valve; (162, 163) sculpture.