368 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [May 10, 



caudal fin, is one of great value and significance in the determination 

 of various genera of fossil fishes, it is nevertheless necessary, in 

 drawing general conclusions, to be careful not to assign to it more 

 importance than it is strictly entitled to ; for we find, by the compa- 

 rison of several genera, that it is not one of those well-defined tren- 

 chant characters which can be affirmed to exist or not, as the case 

 may be, but that it is variable in amount, passing from extreme 

 heterocercy to absolute homocercy by a sliding-scale so gradual, that 

 it is (at all events in fossil examples) most difficult to define a posi- 

 tive line of demarcation between the two forms. 



The researches of one of the most distinguished naturalists in Eu- 

 rope, Professor Muller, tend to show that as much difficulty pre- 

 vails in determining this point in recent fishes ; and in his valuable 

 paper on the structure and characters of the Ganoidei, communicated 

 to the Academy of Sciences at Berlin in December 1844, he comes 

 to the conclusion that heterocercy, anatomically considered, passes 

 insensibly into homocercy . As a proof of this fact, he instances the 

 Sturgeon among Ganoid fish ; which, although considered to be hete- 

 rocerque, is shown nevertheless, on dissection, to have a series of in- 

 articulate cylindrical spines in the upper lobe of the tail attached to 

 other unarticulated spines, placed upon, or on the upper surface of, 

 the chorda dorsalis, of similar structure to those given off from the 

 under surface. So also in Plagiostomi he finds, on dissecting a hete- 

 rocercal Shark, a small rudimental fin above the vertebral column, 

 corresponding in structure to, although of shorter dimensions than, 

 that proceeding from the lower portion of the column. 



As, however, these appearances may be classed with numerous 

 other anatomical facts now attributed to an arrest or interruption at 

 various stages of foetal development, attention has naturally been 

 directed to the evidences aff'orded by a close investigation of embryo- 

 logical phsenomena with reference to the organization of the distal 

 extremity of the vertebral column and its appendages. One of the 

 most elaborate works hitherto published on this subject is that 

 undertaken by M. Vogt, at the instigation and with the advice of 

 Professor Agassiz, on the embryology of the Coregoni, in which it is 

 clearly demonstrated that the tail of the foetal fish, in passing from 

 the embryonic to the perfect state, undergoes a gradual transition 

 from the heterocerque to the homocerque condition. 



In addition to the facts so beautifully detailed and figured in this 

 publication, I am informed in a letter recently received from Professor 

 Agassiz, that he has ascertained some further points of high import- 

 ance in the investigation of the embryology of the genus Lepidotus, 

 which I trust may soon be made public. 



These remarks have been instigated by the examination of a most 

 singular fossil fish lately discovered by the active and intelligent 

 collector of the Geological Survey in the upper strata of the New 

 Red Sandstone at Bromsgrove, and which I now proceed to de- 

 scribe. 



