VII. Median and Paired Fins, a Contribution to the His- 

 tory OF Vertebrate Limbs. By James K. Thacher. 



Median Fins in Amphioxus. 



The quadrate markings seen at the base of the median fin in 

 Amphioxus extend on the dorsal side from one extremity of the 

 animal to the other, or nearly so, and on the ventral side from the 

 porus abdominalis aboi-ad toward the extremity of the tail. They 

 are largest and most distinct in the middle of the body, and become 

 smaller and less clearly marked (as seen from the outside), toward 

 the head and tail until they seem to fade out entirely as they closely 

 approach these extremities. 



As Stieda* has shown these are but the external marks of a series of 

 cavities, containing what is described as " a transparent, wholly struc- 

 tureless mass, resembling a coagulation." 



Thus the relation of these bodies to the somewhat similarly placed 

 primordial tin rays, or "interneural spines," of the Craniote fishes is 

 not so simple and direct as indicated in the mistaken repi-esentations 

 of Rathkef and of Mtiller.J Still they occupy a position similar to 

 that of the primordial fin rays of other fishes, and the fact that they di» 

 not agree with the segmentation of the lateral muscles, seems to have 

 some pertinency here, and to this alone I wish to call attention. 



In the middle of the back there are about five of these bodies to a 

 single segment, and on the ventral side just aborad of the abdominal 

 pore there are about four to each. As we shall see hereafter, the 

 structures of the median line (genuine fin-rays except in Amphioxus) 

 exhibit quite generally throughout the fishes, a total disregard of 

 the segmentation of the lateral muscles, and are more numerous than 

 those segments. 



* Studien iiber den Amphioxus lanceolatus von Dr. Ludwig Stieda, Mem. de I'Acad. 

 Imp. des Sciences de St. Petersbourg, VII^ Serie, Tome xix, No. 7. 



f Rathke, Bemerkungen liber den Ban des Amphioxus lanceolatus. Konigsberg, 

 1841. 



X Johannes Miiller, Ueber den Bau and die Lebensersclieinungen des Branch iosioma 

 lubricum. Abhandl. der Berliner Academic, 1842. 



Trans. Conn. Acad., Vol. III. 36 February, 1877. 



