014 MR. H. 11. \VllTTi:iIOi;SK ON- TIIK [Api. i', 



tlio axis, anil cU'.uiin'- ii<;;eiit.s show, in this tyiK', an ()hli(|ne line 

 of fusion in t.lie ]ti'oxini:il end of this recti nyular lione ; tlii« 

 suii;>i[ests that tlie distnl end is hypnral, nnil the proxiniiil, at lenst 

 l)ai-tly, is nrostyle hrnily fused with the liypiiral. It ni;iy be so 

 in Scomber too, but the fact tliat tlie si)inal cord enters the 

 jiroxiinal end suggests that a neural arch is also involved. And 

 this seems to be the interpretation, for it cannot be urostyle 

 alone, since it is cut square, tij)ped with cnrtihige and bears 

 deruiotrichia just as an ordinary hypural does. It seems safe, 

 therefore, to say that the urostyle is greatly reduced and fused 

 entirely with the last hypux'al— in this case a small one. In all, 

 tlien, four- hypurals ai'e associated with the last vei'tebi'al segment ; 

 the most anterior being provided with a ridge for the attachment 

 of muscles. Two dorsal caudal radials are i)resent. The penulti- 

 mate vertebra possesses a I'educed neui-al arch and a well-developed 

 hvpural, while the antepenultimate vertebra bears an epui'al and 

 a, hypural bone. All the hypurals are iirmly attached proximally ; 

 the dermotrichia embrace tlie terminal endoskeletal supports to 

 their base ; thus the fin forms a most efficient oi'gan of locomotion, 

 seen to advantage in the swift swinnning Mackerel. 



Zeus faber. (Zeorhombi, Zeida\) (Plate XLIX. fig. 23.) 



In this fish, the John Doiy, a fairly typical flat-fish foi'm is 

 met with. The urostyle is lost to view ; clearing shows it to be 

 ])resent as a minute hook embedded in the tissue of the last 

 hy[)vu-al. The three terminal hypurals {hy.) are fused with the 

 last centrum {l.v.) into a single piece, though the outlines of 

 the hypurals still remain visible. The last two complete centra 

 each have a well-developed epural {e]).) and hypural bone, and 

 the former are s})ecially interesting inasmuch as tliey show clearly 

 that the epural is formed by a fusion of I'adial with neural spine, 

 ajiparently^ by means of a secondar-y ossification. The last centrum 

 has a small neural arch {ii.a.) developed. Of other supports of 

 dermotrichia there are two dorsal caudal radials (d.c.r.) between 

 the last epural and hypural, and also a ventral caudal radial 

 {v.c.r.) immediately anterior to the large terminal hj'jiural plate. 

 The caudal fin, therefore, is a specialized homocercal type. 



SoLEA LUTEA. (Zeorlioiubi. rieuronectida'.) (Plate XLIX. 

 iig. 24.) 



Soha liitea may be taken as possessing a good re]ire.sentativo 

 typo of Pleuronectid cauilal fin. It has a more delicate Iniild, 

 but essentially is very similar to that of Zeus. A cU'aring agent 

 sliows the otherwise invisible urostyle to be a iiicic hook-like 

 upwardly directed termination of the last centiuui. The four 

 hvpurals of this centrum are firndy miited, though less so than 

 in Zeas, and occuj^y a terminal position. The penultimate vertebra 

 ha.s a well-developeil epural ('7?.) and hypural {//>/.) bone ; since 

 ri'feii'nci' ha.s been m.ide iiy ("olr to the corresponding epural and 

 hvpural in llic riaicr. it i.^ iiil crest ing to compai'e t-lieni in the 



