箭石类
箭石類動物（學名：），舊作箭石下綱，是一個已滅絕頭足綱生物的分支，生活在泥盆紀至白堊紀之間。箭石在許多方面都與現代的魷魚相當接近，而且與現代的烏賊關係密切。箭石也擁有墨囊，不過牠們有10條大約相同長度的觸腕，其中並沒有特別長的觸腕。
箭石的學名"belemnoid"是來自於希臘文βέλεμνον（"belemnon"，意指「飛鏢或箭頭」），以及另一希臘字είδος（"eidos"，意指「形式」），即「箭頭形狀」的動物。
箭石類動物包括了箭石目、閉箭石目、溝箭石目和Diplobelida。
箭石類在侏羅紀與白堊紀時數量相當多，所以牠們的化石在中生代的海洋岩層中相當豐富，經常伴隨著菊石類出現。不過箭石也跟菊石一起在白堊紀末期滅絕了。箭石是由桿石於泥盆紀時期演化而來，從密西西比紀（或早石炭世）至白堊紀末之間的地層中可以發現良好的箭石化石，其他頭足類如桿菊石、鸚鵡螺類及稜菊石類等也可發現。
Belemnoids possessed a central phragmocone made of aragonite and with negative buoyancy. To the rear of the creature was a heavy calcite guard whose main role appears to have been to counterbalance the front (towards the head) of the organism; it positions the centre of mass below the centre of buoyancy, increasing the stability of the swimming organism. The guard would account for between a third and a fifth of the length of the complete organism, arms included.
Like some modern squid, belemnoid arms carried a series of small hooks for grabbing prey. Belemnoids were efficient carnivores that caught small fish and other marine animals with their arms and ate them with their beak-like jaws. In turn, belemnites appear to have formed part of the diet of marine reptiles such as Ichthyosaurs, whose fossilized stomachs frequently contain phosphatic hooks from the arms of cephalopods.
Belemnoids were effectively neutrally buoyant, and swam in near-shore to mid-shelf oceans. Their fins could be used to their advantage in all water speeds; in a gentle current they could be flapped for propulsion; in a stronger current they could be held erect to generate lift; and when swimming rapidly by jet propulsion they could be tucked in to the body for streamlining.
箭石形成化石的部份大多是其結實的「護甲」，通常呈拉長的子彈形。(though in some subgroups the rostrum may only exist as a thin layer coating the phragmocone).The hollow region at the front of the guard is termed the "alveolus" 護甲後端的中穴內包覆或連接著圓錐狀的，而閉錐就是箭石調節浮力的氣室，大多數箭石標本的護甲末端中穴與閉錐部份常會被壓碎，沒有完整的保存下來，只有保存較好的標本能保留下來。Projecting forwards from one side of the phragmocone is the thin "pro-ostracum".
While belemnoid phragmocones are homologous with the shells of other cephalopods and are similarly composed of aragonite, belemnoid guards are evolutionarily novel and are composed of calcite or aragonite, thus tending to preserve well. Broken guards show a structure of radiating calcite fibers and may also display concentric growth rings.
The guard, phragmocone and pro-ostracum were all internal to the living creature, forming a skeleton which was enclosed entirely by soft muscular tissue. The original living creature would have been larger than the fossilized shell, with a long streamlined body and prominent eyes. The guard would have been in place toward the rear of the creature, with the phragmocone behind the head and the pointed end of the guard facing backward.
The guard of the belemnoid "Megateuthis gigantea", which is found in Europe and Asia, can measure up to in length, giving the living animal an estimated length of .
Very exceptional belemnoid specimens have been found showing the preserved soft parts of the animal. Elsewhere in the fossil record, bullet-shaped belemnite guards are locally found in such profusion that such deposits are referred to semi-formally as "belemnite battlefields" (cf. "orthocone orgies"). It remains unclear whether these deposits represent post-mating mass death events, as are common among modern cephalopods and other semelparous creatures.
在數百年前的歐洲，尚未有古生物學概念的時代，當時的英國人認為箭石的化石是雷打到地面而形成的，所以稱其為「雷石（Thunderstones）」，成為箭石化石的別稱。
The 稳定同位素 composition of a belemnoid rostrum from the Peedee Formation (Cretaceous, southeast USA) has long been used as a global standard (Peedee Belemnite, "PDB") against which other isotope geochemistry samples are measured, for both 碳同位素 and 氧同位素.
Some belemnoids (such as "Belemnites" of Belemnitida) serve as index fossils, particularly in the Cretaceous Chalk Formation of Europe, enabling geologists to date the age the rocks in which they are found.