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a family of dolphins of the suborder Odontoceti. The dolphins reach a length of 3 m and a weight of 225 kg. There are individual hairs on the snout. The pectoral fins are short and wide, and the dorsal fin is shaped like a low triangular ridge. The beak is very long and narrow, and the numerous teeth (from 104 to 242) have curved and broadened roots. The front teeth of the Ganges dolphin (Platanista gangetica) are particularly large. Hearing and echolocation are well developed, but eyesight is poor. The Ganges dolphin has no crystalline lens.
|Table 1. Comparative indexes of methods used to work placers in the USSR|
|Dredges||Scrapers, bulldozers, and excavators||Hydraulic machinery|
|Density of volumes of extraction and washing of productive rocks (percent)....................||70–75||12–16||8–11||3–5|
|Average content of valuable component per cu m of productive rocks (units by weight)..............||1||5–8||1–2||15–20|
|Net cost of extraction and washing of productive rocks (cost units)......||1.0||6.5–8.0||1.5–2.0||20–30|
The Platanistidae inhabit the rivers of South America and South Asia. There are four genera, each having a single species. Geoffroy’s dolphin (Inia geoffrensis) lives in the rivers of South America, and the Ganges dolphin is found in the Ganges and Brahmaputra rivers. The Chinese dolphin (Lipotes vexillifer) inhabits Lake Tungting, and the La Plata river dolphin (Pontoporia blainvillei) is found in the Rio de la Plata and off the shores of Brazil, Uruguay, and Argentina between 30° and 45°Slat.
The dolphins feed on bottom-dwelling invertebrates and fish. The gestation period of the Ganges dolphin is eight to nine months.
A. G. TOMILIN