论Paracarinachitids

研究证明Paracarinachitids的外骨骼都不是由8个骨板组成,骨片具有清楚的微层构造,表明它们是由叠覆的生长层组成,每个生长节上有一个小刺,在顶端下侧面前端都有肾形缺凹。同一标本上存在顶端部的融合和远顶端部的分节彼此递交的现象表明这种表象上的差异是后期次生磷灰石包裹或腐蚀的结果;其下侧缘融合和下侧面无缝隙的现象说明将Paracarinachites spinus的骨片分成头板、中间板和尾板是没有根据的。骨片表面存在的针状体是成岩作用的产物,它是粘土矿物和磷酸盐针状结晶体而不是多板纲的骨针。凡此强有力地说明Yangtzechiton是Paracarinachites的晚出同义名。根据形态功能、组织结构,特别是叠覆状排列的骨片和孤立的生长节等特征,paracarinachitids 可能与zhijinitids有密切关系,但两者是否同源尚缺少进一步的证据、据此,笔者认为将paracarinachitids 暂置于分类位置未定的具骨片后生动物较妥。

ON PARACARINACHITIDS

Studies have proved that the sclerites of paracarinachitids are not all composed of 8valves,and the elongated sclerites are formed by the successive incremental growth in theapical laminated structure. Each growth increment has a spine,with a nephroid depression atthe anterior margin on the lower side of the sclerite. The fusion at the apical part and the in-crement at the adapical part on the same specimen suggest that such superficial differences resulted from secondary apatite coating or physical abrasion,while the lateral fusion and theseamless appearance on the lower side suggest that there is no ground for dividing the scle-rites of paracarinachitids into head valve,mediate valve and tail valve. The spicules present onthe surface of sclerites are products of the diagenetic stage. They are the needle--shaped crys-tallites of clay mineral or phosphorites, different from the spicules of the Polyplacophora. Inconclusion,the author considers Yangtzechiton together with Luyanhaochiton as a junior syn-onym of Paracarinachites; moreover,up to now no evidence has been found to prove its attri-bution to the Polyplacophora. According to morphological function, composition and struc-ture, particularly the overlapping sclerites and the isolated growth increments, the paracari-nachitids may be related to the zhijinitids. However, further data are needed to prove whetherboth of them are homologous. Therefore,the author perfers to place the paracarinachitids inthe sclerite--bearing metazoans which are still uncertain in taxonomical position.