pointed protuberance(凸起) on its head；its purpose is unknown.
There are about 368 different species of sharks，which are divided into 30 families.These different families of sharks are very different in the way they look，live，and eat.They have different shapes, sizes, colors, fins, teeth, habitats, diets, personalities, methods of reproduction，and other characters.Some types of sharks are very rare(like the great white sharks and the megamouth sharks) and some are quite common(like the dogfish sharks and bull sharks)．
【写作指导】
要写好说明文类文章的概要，主要需把握好以下几点：(1)本文介绍和说明的核心事物是什么？(2)该事物的主要特征是怎样的？(3)该事物的用途有哪些等。就本文而言，其概要框架应该是：第一段引出本文介绍的主要事物——鲨鱼；第二至五段从四个方面介绍鲨鱼的主要特征：骨架、尺寸、体形、种类等。本文属于总分结构，因此学生应注意在概要中也要体现这一结构特点，各要点间可用过渡衔接词和连接词将其串联起来，以使概要连贯、有条理。 【参考范文】
According to the passage，we can learn that sharks are amazing fish living in the world.(要点1) Sharks have no real hard bones but cartilage，and there are different species of sharks of different sizes，all of which can fly easily through the water.(要点2) Besides，sharks have various body shapes.(要点3) In all，about 368 different species of sharks live in waters，some types of which are very rare.(要点4)
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