532 Mr. C. T. Regan on the Caudal Fin 



Chatoessus there are only four supporting the upper lobe of 

 the fin, whereas there are five in Elops. The third hypural 

 of the lower lobe (marked 7 in the figure of Chatoessus) is in 

 each case attached to a centrum, to which there is also united 

 a pair of elements which I term " anterior uroneurals," the 

 union being by suture in Elops and by ankylosis in Chato- 

 essus ; these anterior uroneurals are strong bones, directed 

 upwards and backwards, and can be readily separated from 

 each other right down to their junction with the centrum; 

 they embrace a laminar bone, the paired nature of which is 

 sometimes evident, and which represents, in my opinion, the 

 neural arches of centra behind the one which bears the ante- 

 rior uroneurals ; to the posterior edge of the anterior uroneurals 

 is attached a second pair of bones, posterior uroneurals, and a 

 third smaller distal pair is also present in both genera. In 

 Elops the uppermost hypural of the lower caudal lobe is 

 attached to the centrum behind that which bears the anterior 

 uroneurals ; in Chatoessus this hypural (marked 5 in the figure) 

 has probably ankylosed with its centrum, which is not present 

 as a separate element. The next hypural (4) is articulated 

 with the last centrum in Elops, and in Chatoessus with a 

 small median element (c) which obviously represents the last 

 centrum ; this sends up a spur-like process on each side of 

 the base of the hypural above it, these processes corresponding 

 to similar prolongations of the last centrum in Elops, pro- 

 tecting the sides of the notochord and concealed beneath the 

 posterior uroneurals. 



Other Clupeidse do not differ very much from Chatoessus ; 

 the last centrum may not be recognizable as a distinct 

 element, and the epurals may be reduced in number to two, 

 but on the whole the variation is comparatively unimportant. 



A detailed comparison shows that the Clupeidae differ from 

 the Elopidaj in 



(1) ankylosis of the neural and hasmal arches with the 



centra ; 



(2) ankylosis of the anterior uroneural with the third 



last centrum ; 



(3) disappearance of the penultimate centrum as a sepa- 



rate element ; 



(4) reduction or abortion of the last centrum ; 



(5) reduction in number of the hypurals of the upper 



caudal lobe from five to four. 



Thanks to the courtesy of Mr. E. T. Newton I am able to 



o-ive a figure of the caudal fin skeleton of a young Clupea, 



which he has already described and figured (Journ. Quekett 



Micr. Club, (2) i. 1882, p. 82, pi. iii. fig. 1). I have nothing 



