232 Transactions. — Zoology. 



rather with " L." The rnid-brain (fig. 13, m.b) forms the anterior termina- 

 tion of the head, and the ventral surface of the fore-brain (f.b) looks directly 

 backwards. 



The mouth (fig. 14, m) is greatly reduced, and has in fact almost pre- 

 cisely the form, relative size, etc., as in Balfour's "0." So also have the 

 branchial apertures (br. 1, br. 5), from which as well as from the spiracle 

 (sp) the long external gills now emerge. The dorsal (ell, d 2) and pectoral 

 (pc) fins are beginning to assume their adult form, their line of attachment 

 being no longer their greatest dimension. The caudal fin (c) shows the first 

 indication of the change from diphy- to hetero-cercality : on its ventral 

 edge, near the tip, is a slight emargination, evidently the commencement of 

 the very marked notch in the corresponding position in the adult (fig. 15). 

 In front of the anterior end of the pelvic fin (pv) the lateral ridge is now 

 visible and has the same relations and proportions as in the adult (fig. 15, 

 l.r). The length of the embryo in this stage is about 40 mm. 



Up to the present time I have been able to do very little towards the 

 further examination of these embryos, and all I propose to bring forward in 

 the present paper is the fact that in the third stage the lateral vein (figs. 5 

 and 6, l.v) is well developed, and is indeed nearly as large as the cardinal 

 vein (cd), and considerably larger than the dorsal aorta (d.ao). Of the two 

 sections figured, fig. 5 is taken along the line xy in fig. 13, or just in front 

 of the pelvic fins, fig. 6 along x'y' or through the pelvic fins. In both 

 figures the lateral vein (l.v) is seen to be a very obvious structure, a fact 

 which makes its absence in Scyllium and Pristiurus — since it is not shown in 

 any of Balfour's figures — somewhat remarkable. Having, unfortunately, no 

 spirit specimen of either of these genera, I have not been able to ascertain 

 whether it exists in the adult : as mentioned above, however, it is present 

 in the allied Chiloscyllium. 



A few weeks ago I received a letter from Mr. Balfour,* in which the 

 following passage occurs : — " I was very much interested in your paper on 

 the skate's venous system. The lateral veins you describe are very peculiar, 

 and I should not hesitate to consider them as confirming my view of the 

 fins, were it not for the specialized character of the skate, which you your- 

 self urge in your paper. One would like to find them either in the embryo 

 or in some less specialized form." 



The necessary confirmation is afforded by the facts detailed in the 

 present paper. The lateral vein exists in every Selachian I have yet had the 

 opportunity of examining : in all it follows the direction of Balfour's lateral 

 ridge, from the anterior border of the pelvic fin forwards and upwards to 



* I little thought at the time that this letter was the last I was ever to have from the 

 writer. 



