IV. On Chondrosteus,an extinct genus of fish allied to the sturionidæ

P. D. M. Grey Egerton
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Abstract

Before the conclusion of his great work on Fossil Fishes, Pro­ fessor Agassiz recognized in some fragmentary remains found in the lias strata at Lyme Regis, unmistakeable evidence of the existence, at that period of the earth’s deposition, of a representative of the still extant family of the Sturgeons. To this extinct fish he assigned the name Chondrosteus. The author of the present memoir has been enabled, by the examination of numerous specimens more re­ cently acquired, to describe in some detail the external features of the fish, and the structural peculiarities of those portions of the exoand endo-skeleton which have been preserved. In the former respect the fossil differs from the recent sturgeon in having a shorter and deeper trunk, in the greater vertical expanse and wider diver­ gence of the lobes of the caudal fin, in the median position of the dorsal fin, and in the absence of dermal plates on the back, belly, and flanks. Before describing the cranial anatomy, the author points out certain homologies between the head-plates of the recent sturgeon and the epicranial bones of the teleostean fishes, more especially with reference to the parietals, mastoids and frontals ; and explains that these conclusions have resulted from the examination of the inner table of skull, where the relative position and propor­ tions of the component plates are constant, however much the outer or dermal layer may vary. The remainder of the memoir is devoted to detailed descriptions of such parts as are preserved in the several specimens; and the author concludes by stating as the result of his investigations, that Professor Agassiz was right in referring the liasic fish to the Sturionidse; that in some respects it evidenced a transitional form between the latter family and the more typical ganoids; that its food was similar to that of the existing members of the family, but that it was procured in a tranquil sea, rather than in the tumultuous waters frequented by sturgeons of the present time.
关于软骨鱼,一种已灭绝的鱼属,与鲟鱼科有亲缘关系
在他关于鱼类化石的伟大著作结束之前,阿加西斯教授从莱姆里吉斯海底地层中发现的一些残片残骸中发现了确凿的证据,证明在地球沉积的那个时期,存在着一种仍然存在的鲟鱼科的代表。他把这种灭绝的鱼命名为软骨鱼。本回忆录的作者,通过对最近获得的许多标本的研究,已经能够比较详细地描述这种鱼的外部特征,以及保存下来的外骨骼和内骨骼部分的结构特点。就前者而言,该化石与现代鲟鱼的不同之处在于,它的躯干更短、更深,尾鳍裂片的垂直延伸更大、垂度更宽,背鳍位于中间位置,背部、腹部和两侧没有皮板。在描述颅骨解剖之前,作者指出了近代鲟鱼的头板与硬骨鱼类的颅外骨之间的某些同源性,特别是在顶骨、乳突和额骨方面;他解释说,这些结论是由对颅骨内表的检查得出的,在那里,各组成板的相对位置和比例是恒定的,然而外层或真皮层可能会有很大的变化。回忆录的其余部分详细描述了保存在几个标本中的这些部分;作者最后说,根据他的调查结果,阿加西斯教授把这种有刺的鱼和刺鱼联系在一起,这是对的;在某些方面,它证明了后一科和更典型的类目动物之间的过渡形式;它的食物与现存的鲟鱼家族成员的食物相似,但它是在平静的海洋中捕获的,而不是在现在鲟鱼经常出没的喧嚣水域中捕获的。
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