NARRATIVE OF THE iRUISE. 
933 
The size of the frustules of this genus varies very greatly. Etmodiscus humilis and 
Etmodiscus obovcttus are among the smallest forms hitherto recorded, Etmodiscus convexus 
measures 123 g , 1 Etmodiscus punctiger 143 g, while Etmodiscus gigas reaches 1633 g, 
and Etmodiscus spheroidcdis 1882 g. 
The general shape of the frustule varies with the development of the connecting zone, 
and the degree of convexity of the valves. Four leading forms, however, may be 
recognised, namely, the discoid, as in Etmodiscus convexus, Etmodiscus radiatus, &c.; the 
oval, as in Etmodiscus obovatus ; the cylindrical, as in Etmodiscus wyvilleanus (see 
fig. 332) and Etmodiscus tympanum ; and the compressed spheroidal, as in Etmodiscus 
spheroidcdis. In Etmodiscus tympanum a noteworthy 
superposition of a belt to the two hoops of the valves 
occurs, as in the genus Biddulphia, and in Etmo- 
discus spheroidcdis the belt exhibits a small sutural 
line in its middle part, recalling in some measure that 
of Melo'sira and of Poclosira. 
The geographical range of the genus Etmodiscus 
is very extensive. Species occur in the Arafura Sea 
(e.g., Etmodiscus convexus) and the Sea of Japan 
(e.g., Etmodiscus japonicus). Etmodiscus humilis 
was found in mud procured from a sounding of 
1950 fathoms near the ice barrier of the Antarctic 
Ocean, and Etmodiscus diadema was also obtained 
in the same ocean to the south of Heard Island. On the other hand, Etmodiscus gigas, 
Etmodiscus wyvilleanus, and Etmodiscus spheroidcdis are Atlantic and Pacific species, 
and occur everywhere at the surface of the ocean within the tropics. 
Infusoria. — Noctiluca miliaris was observed -in great numbers in the water of 
harbours, but was rarely met with in the open sea far from land. It was abundant 
at Vigo in Spain, at the Cape of Good Hope, in several parts of the Philippines, at 
Hong Kong, at Japan, and off the coast of South America. On only two occasions were 
specimens taken at a great distance from the coast — once in the Japan Stream and once 
off the north coast of New Guinea — and in both instances the water was of low specific 
gravity and gave other evidences of being coast water ; the specimens were also con- 
tracted and shrivelled up. 
Species of Carchesium, Vorticella, Acineta, Podoplirya, Tintinnus, and Ceratium arc 
all much more abundant in the waters of harbours, enclosed seas, and coast waters 
generally than in the open ocean. Latterly, indeed, when many representatives of these 
genera, together with Noctiluca, certain other Infusoria, Ilydromedusae, and numerous 
1 ^ = 0-001 millimetre. 
Fio. 333. — Fragment of frustule of 
Etmodiscus sp. its. 
