Parker. — Anatomy and Embryology of Scymnus lichia. 227 



and slightly upwards, parallel with the cutaneous ridge described above, to 

 the pectoral fins, where it is connected with the brachial veins. It thus 

 marks exactly the position of Balfour's lateral ridge, or in other words of 

 the hypothetical ancestral lateral fin. It is worthy of notice that the body- 

 muscles are disposed peculiarly with regard to this vein (fig. 4), a transverse 

 section showing that the muscular bundles are disposed around it in a radi- 

 ating fashion. The section also shows that the ridge in question is not a 

 mere cutaneous structure like the lateral keels on the tail of Lamna, Car- 

 ckarodon, etc., which are formed merely by a thickening of the tough, 

 white, fibrous tissue of the dermis. In the pre-pelvic ridge of Scymnus, on 

 the other hand, the skin is no thicker than in other parts, but is moulded 

 on an actual muscular ridge. Throughout the greater part of its course 

 the lateral vein lies immediately beneath the peritoneum. 



Physiologically, I am disposed to think that the lateral vein has but 

 little significance, since except at its anterior and posterior ends it receives 

 only the small veins from the abdominal walls. This, coupled with the 

 structural peculiarities just mentioned, seems to confirm the view I advanced 

 in describing the corresponding veins in the skate, namely, that the lateral 

 vein represents the vein of the primitive vertebrate lateral fin. It seems 

 possible also that it may be genetically derived from the lateral vessel of a 

 more remote vermian ancestor, but this is merely a suggestion. 



The lateral vein exists also in Acanthias vulgaris, Mustelus antarcticus, 

 and Chiloscyllium furvum, in which, as in Scymnus, it is so obvious a struc- 

 ture that, in spite of the absence of any mention of it in the books at my 

 disposal, I feel sure it must have been previously noticed. 



4. Urinogenital organs. 



The kidneys are very long, extending nearly to the anterior boundary 

 of the body-cavity, and apparently representing both meso- and meta- 

 nephros. That the mesonephros should remain functional in the adult female 

 is noteworthy, since from the analogy of other Selachians it is probably con- 

 verted in the male into the epidymis. A single ureter runs alongside the 

 inner edge of each kidney, widening posteriorly, and finally dilating into the 

 urinary bladder. Projecting into the cloaca at its anterior end is an unusually 

 large median urinary papilla (fig. 1, u.p), on the ventral surface of which, 

 near the apex, is the single urinary aperture ; this leads into a compara- 

 tively narrow canal in the very thick-walled papilla, and into the anterior 

 end of the urethral passage thus constituted the two urinary bladders open. 



The oviducts open into the cloaca by widish apertures (fig. 1, ut'), one 

 on either side of the urinary papilla. The posterior part of each (ut) is 

 wide, having in the gravid state a considerably greater diameter than either 

 the stomach or intestine, and forming a uterus or brood-pouch : a little 



