门: 节肢动物门 von Siebold, 1848
纲: 三叶虫纲 Walch, 1771
目: 球接子目 Salter, 1864
科: 球接子科 M’Coy, 1849
属: 球接子属 Brongniart, 1822
种: 豆荚形球接子 Wahlenberg, 1818
描述:
背部外骨骼：背甲分为1个头甲，2个胸甲与1个尾甲。头甲与尾甲均为马蹄形且具有相似的轮廓、凹凸与高度。头甲后方与尾甲前方均被截断。头甲与尾甲中部凸起，包括头甲上的“头鞍”与尾甲上的“轴”。在头甲与尾甲各部分融合的过程中，大部分外部节段在进化过程中被舍弃。特别是肋部区域与边缘几乎不可见分节。只有头甲后部与尾甲前部表面保留些许原始分节结构特征。此外，轴部也因 “横向轴沟”的存在而被分为多段。肋部与轴部均以“轴沟”相隔。前头鞍区的中沟位于轴沟与边缘沟之间。头甲与尾甲的边缘向内强烈弯曲，形成内衬。上下部分轴叶三分不明显。两个胸节的轴环发育且三分明显。头甲与尾甲后部具有成对的侧刺。
附肢：成虫阶段共发育9对附肢：4对位于头部，5对位于躯干部。触角后部前两对附肢与后部附肢差异明显，主要在于附肢基部的差异：第二与第三对头部附肢外肢更长也更为突出，然而内肢却发生退化；第四附肢与所有其它躯干附肢具有巨大内肢，而外肢却相对较小。
Phylum: Arthropoda von Siebold, 1848
Class: Trilobita Walch, 1771
Order: Agnostida Salter, 1864
Family: Agnostidae M’Coy, 1849
Genus: Agnostus Brongniart, 1822
Specie: Agnostus pisiforms Wahlenberg, 1818
Description:
Dorsal exoskeleton: The tergum is divided into a cephalic shield, two tergites forming a short thoracic region, and a pygidial shield. Both shields are horseshoe-shaped and of about the same outline, convexity, and height. The cephalic shield is truncate posteriorly, while the pygidial shield is truncate anteriorly. It consists of an arched area medially, the 'glabella' on the cephalic shield and the 'axis' on the pygidial shield. In the course of fusion of the cephalic and pygidial segments and their tergal cuticle, most of the external segmentation has been reduced during evolution. In particular the pleural areas and borders are almost devoid of the original segmental surface relief. Only the posterior-most cephalic and the anterior-most pygidial segments have retained parts of the original segmental surface structures. Furthermore, the axes are more or less divided into a number of lobes by 'transverse axial furrows'. The axes are separated from the pleural areas by 'axial furrows'. On the cephalic shield a furrow, the 'preglabellar median furrow' expands between the axial furrow and the ' border furrow', which marks off the border from the genal fields. The margins of the two shields are strongly flexed inwards, forming the 'doublures'. On the shields the originally tripartite nature of the 'axial lobes' or 'rings' is less defined or absent; on the two tergites the axial rings are, well developed and tripartite. Both shields have pairs of posteriorly directed spines.
Appendages: Nine pairs of appendages are developed in the holaspid stage: four cephalic pairs and five pairs in the trunk region. The two pairs of post-antennular appendages can be clearly differentiated from the more posterior limbs, mainly on the basis of the shape of the rami: in the second and third cephalic limbs the prominent part is the long exopodite, while the endopodite is reduced or absent; in the fourth and last cephalic limb and in all trunk limbs the main axis forms the large endopodite, while the exopodite is very short.
时代：
晚寒武世，豆荚形球接子三叶虫带。
主要产地:
欣讷山，西约特兰，瑞典南部
简要描述：
典型球接子；具有附肢；豆荚状
Age:
Late Cambrian, Agnostus pisiformis Zone.
Principal localities:
Kinnekulle, Västergötland, southern Sweden.
Brief description:
Typical Agnostida; Appendages bearing; Pea-like.
Müller, K.J., Walossek, D., 1987. Morphology, ontogeny, and life habit of Agnostus pisiformis from the Upper Cambrian of Sweden. Fossils Strata 19, 1–124.
Eriksson, M. E., & Horn, E. (2017). Agnostus pisiformis—a half a billion-year old pea-shaped enigma. Earth-Science Reviews, 173, 65-76.
Jackson, I. S., & Budd, G. E. (2017). Intraspecific morphological variation of Agnostus pisiformis, a Cambrian Series 3 trilobite‐like arthropod. Lethaia, 50(4), 467-485.