［A］The shellfish have a long history in the rock and many different kinds are known.
［B］ Nevertheless, we know a great deal about many of them because their bones and shells have been preserved in the rocks as fossils. From them, we can tell their size and shape, how they walked, the kind of food they ate.
［C］The first animals with true backbones were fishes, first known in the rocks of 375 million years ago. About 300 million years ago the amphibians, the animals able to live both on land and in water, appeared. They were giant, sometimes 8 feet long, and many of them lived in the swampy pools in which our coal seam, or layer, formed. The amphibians gave rise to the reptiles and for nearly 150 million years these were the principal forms of life on land, in the sea, and in the air.
［D］The best index fossils tend to be marine creatures. These animals evolved rapidly and spread over large areas of the world.
［E］The earliest animals whose remains have been found were all very simple kinds and lived in the sea. Later forms are more complex, and among these are the sea-lilies, relations of the star-fishes, which had long arms and were attached by a long stalk to the sea bed, or to rocks.
［F］When an animal dies, the body, its bones, or shell, may often be carried away by streams into lakes or the sea and there get covered by mud. If the animal lived in the sea its body would probably sink and be covered with mud. More and more mud would fall upon it until the bones or shell become embedded and preserved.
［G］Many factors can influence how fossils are preserved in rocks. Remains of an organism may be replaced by minerals, dissolved by an acidic solution to leave only their impression, or simply reduced to a more stable form.
 
解題思路：
本文文章部分共332字，備選答案部分字?jǐn)?shù)為317字，總數(shù)達(dá)649字。閱讀如此長的文章并做對題，要求考生具有較強(qiáng)的閱讀理解能力，掌握正確的閱讀方法。
首先，通篇略讀原文，把握要點(diǎn)詞句（文中黑斜體部分為要點(diǎn)詞句，以下同），以便對文章主旨、各段基本主題以及邏輯結(jié)構(gòu)獲得一個(gè)概括性了解。
第一段指出：在人類出現(xiàn)很早以前，地球上就出現(xiàn)了魚類、爬行動(dòng)物、鳥類和一些哺乳動(dòng)物（這是對全文的一個(gè)概述，點(diǎn)出本文的話題）。雖然這些動(dòng)物有一些是現(xiàn)今生活著的各種動(dòng)物的祖先，但是另一些動(dòng)物現(xiàn)在已經(jīng)滅絕了，即它們現(xiàn)在已經(jīng)沒有活著的后代了。第二句是個(gè)表示讓步關(guān)系的句子，重點(diǎn)是主句部分：others are now extinct, that is, they have no descendants alive now，而that is表明they have no descendants alive now是同位語結(jié)構(gòu)，因此本段要點(diǎn)詞語為others are now extinct。這為回答第41題從意思和邏輯上都提供了主要依據(jù)。
第二段首句指出：偶爾根據(jù)石頭里所顯示出來的印記，我們可以合理地推斷出幾百萬年前滅絕了的某種動(dòng)物的準(zhǔn)確形狀，盡管不能確定其顏色。此句前后是因果關(guān)系，提到了rocks，結(jié)果是so that…we can build up…picture。后面又接著講rocks in which the remains are found。由此可知，這些是本段的要點(diǎn)詞語。
第三段中已知信息是：我們所知道的所有化石幾乎都是由于水的作用而被保存在了石頭中，而且大多都是生活在水里或水邊的動(dòng)物。因此，肯定存在過許多我們一無所知的哺乳動(dòng)物、鳥類和昆蟲類。空白后第一句是個(gè)并列句，主語分別是Nearly all of the fossils和 most of these (the fossils)，而且都與water action有關(guān)。這為回答第42題提供了主要線索。
第四段中提供的信息是：還有像螃蟹一樣的動(dòng)物，其身體表面是一層角狀物。接下來此段詳細(xì)介紹了這類動(dòng)物的外貌特征。此處要點(diǎn)詞語是also和crab-like creatures，說明上一句也在描述某種海生物。這為回答第43題提供了主要線索。
第五段空白后第一句中有指代詞of these，these必定指代前文提到的復(fù)數(shù)名詞。緊接著提到的ammonites是“這些貝殼類動(dòng)物”的一種。后面有對它的描述：ammonites非常有趣而又很重要。它們身體上有個(gè)由許多腔室構(gòu)成的硬殼（shell），每個(gè)腔室都代表著這個(gè)動(dòng)物的臨時(shí)住所。這樣，上一句提到的自然應(yīng)該與貝殼類動(dòng)物有關(guān)，為回答第44題提供了線索。
第45題為一段。下一段提到，大約7 500萬年前，爬行動(dòng)物時(shí)代結(jié)束了，其中大部分物種都滅絕了。而哺乳動(dòng)物迅速發(fā)展起來，我們可以追尋出許多蹤跡，借此了解許多大家熟悉的動(dòng)物如大象和馬的進(jìn)化過程。后來的許多哺乳動(dòng)物，雖然現(xiàn)在已經(jīng)滅絕了，還是被原始人類所了解，并被畫在了壁畫中或被雕刻在了骨頭上。注意本段第一句提到reptiles。由此我們推斷上一段結(jié)尾處談的內(nèi)容必定與reptiles有關(guān)。這為回答第45題提供了主要線索。另外，最后一段第一句提到了年代，而選擇項(xiàng)C中提到了幾個(gè)年代，說明這里在按歷史時(shí)序敘述動(dòng)物的進(jìn)化過程。
通過這樣閱讀全文要點(diǎn)信息，我們知道了本文結(jié)構(gòu)的脈絡(luò)：首先介紹什么是史前動(dòng)物和人們研究史前動(dòng)物的依據(jù)——化石，以及化石的形成過程。隨后，作者按照動(dòng)物進(jìn)化的順序（最早的動(dòng)物，水生殼類動(dòng)物，脊椎類動(dòng)物等）逐一加以介紹。
當(dāng)然，根據(jù)段落意思、句意和邏輯關(guān)系考查正確答案僅僅是可利用的手段之一，雖然它們提供了主要線索�？忌�還應(yīng)該學(xué)會(huì)對詞語線索的考查，以確保選出的答案在意思和用詞上都與上下文銜接緊湊。有關(guān)重點(diǎn)詞（包括相關(guān)代詞、連詞、名詞或名詞短語、動(dòng)詞等）已在原文中用斜體涂黑標(biāo)出，這里暫且不作具體分析�？傊�，做題時(shí)要十分注意空白前后的句子，仔細(xì)分析句與句之間的關(guān)系，段與段之間的過渡銜接，根據(jù)上下文內(nèi)容線索、邏輯關(guān)系、詞語線索等，來判斷每個(gè)題的正確答案。