1910.] CAUDAL KIN Ob' THE TKLEOSTO.MI. 613 



of the fin-i'ay supports, and here and there are adfh'tional distal 

 cartilages applied to these epiphyses; they may represent the 

 persistent distal segments of radials. 



This caudal fin is a good intermediate form between the lowly 

 and the highly specialized homocercal types. 



Labrus festivus. (Perciformes, Labridtv.) (Plate XLIX 

 fig. 21.) 



No nrostyle can be seeii in this form, it having completely 

 fused with the last hypural bone ; thus the last hypural, urostyle, 

 and last centrum {l.v.) become a single bone. The last centrum 

 bears one other hypural similar in size to the last ; the preceding 

 centrum also bears a hypural bone. Between the hypural of the 

 penultimate vertebra and the first of the last centrum is a large 

 bone (v.c.r.), which on account of its not having actually fused 

 with the centrum, is probably better referied to as a ventral caudal 

 radial. Three bony dorsal caudal radials (d.c.r.) are situated 

 between the neural arch of the antepenultimate vertebra and the 

 last hypural bone. In an exactly corresponding position dorsally 

 and ventrally {i. e. on either side of the distal end of tlie neural 

 spine and hfemal spine of the antepenultimate vertebra) are 

 two cartilages (c.) the posterior of which may in some species, 

 e. g. Labrus turdus, become quite radial-like. The extrenn'ty 

 of the spinal cord is protected along the dorsal side of the last 

 hypural by a secondary bony development [s.). 



Though the structure of this caudal fin approaches closely to 

 secondary symmetry as regards general form, it is really not 

 symmetrical owing to the fact that by far the majority of 

 dermoti'ichia are hypiaxial. By the loss of the urostyle and the 

 reduction of the number of hypurals to the last vertebral segment 

 to two, this caudal fin is a considerable advance in homocercy on 

 the preceding type. 



Scomber scomber. (Scorn briformes, Scombrida?.) (Plate XLIX. 

 fig. 22 ) 



The tail of the Mackerel is somewhat difficult to interpret at 

 first sight as regards the ending of the vertebral axis ; it appears 

 as though a well-marked urostyle w^ere present extending to the 

 length of the broad hypural bones. Examined more closely, 

 doubts will arise as to its true urostylar natui'e for one or two 

 reasons: — (1) its end is cartilaginous and it bears dermotrichia ; 

 (2) its distal end is cut sc^uare, a feature no urostyle elsewhere 

 possesses ; (3) the end of the spinal cord enters slightly into the 

 tissue of the proximal end of the bone. Another feature worth 

 notice is that clearing agents do not reveal any indications of a 

 normal urostyle fused with any other bone, and moreover, the 

 conical last centrum does not turn upwards as a hook or reduced 

 mostyle as so many other forms show, but ends in a. direct 

 straight line. The tail of Trachbms is the only other one 

 examined which closely resembles Scomber in the termination of 



