347 



Scymnus 1 ), and Acanthias 2 ) there is but a single basibranchial 

 cartilage or copula in front of the broad plate, and in all three genera 

 the two hypobranchials of the second branchial arch unite in the mid- 

 ventral line, in Scymnus even fusing to form a continuous transverse 

 bar of cartilage. In Scymnus and Acanthias the third hypo- 

 branchials also meet one another; but while in Acanthias they meet 

 behind the copula, and touch the broad basibranchial plate, they meet, 

 in Scymnus, in front of the copula, and are separated by it from 

 the plate. In Spin ax the copula is elongated and extends from the 

 second hypobranchials to the basibranchial plate, thus separating the 

 third hypobranchials. 



In Laamargus 3 ) there are three basibranchial cartilages in 

 front of the great plate. They are crowded together and lie behind 

 the second hypobranchials, which meet and fuse in the middle line. 

 If the first of these three cartilages is correctly identified as belonging 

 to the second branchial arch, it has suffered a marked backward dis- 

 placement, the normal position for the basibranchial being anterior to 

 the corresponding hypobranchials. A study of White's figure would 

 suggest that the front copula is the basibranchial of the third arch, 

 the hinder one that of the fourth arch, while the intermediate cartilage 

 is a dismembered part of one of the surrounding morphological units; 

 but, since the author had several specimens at his disposal, the one 

 figured is presumably not an individual variation. 



In Echinorhinus (see figure) there are three copulas lying 

 anteriorly to the great plate, and these are approximately equal in 

 size, and are uniformly spaced. They are obviously the mid -ventral 

 units of the second, third and fourth branchial arches, and they lie 

 in front of their respective hypobranchials. The hypobranchials them- 

 selves are also uniformly disposed, and lie transversely to the median 

 line, and gradually diminish in size from before backwards. As usual 

 in Elasmobranchs, there are no fifth hypobranchials, but the fifth cerato- 

 branchials abut directly on the sides of the great basibranchial plate, 

 which may be assumed, in the absence of evidence to the contrary, 

 to represent the basibranchial of the fifth arch. The first hypobran- 

 chials are not constantly present in Selachians; in Echinorhinus 

 they are absent, the antero - mesial extremities of the first cerato- 

 branchials being connected with the basihyal by ligament only. 



The evidence of the degree of specialisation afforded in Sharks 



1) Ibid., Taf. XIX, Fig. 2. 



2) Ibid., Taf. XVIII, Fig. 3. — Rathke, Kiemenapparat der Wirbel- 

 tiere, Riga und Dorpat, 1832, Taf. Ill, Fig. 3. 



3) White, P. J., Trans. B, Soc. Edinburgh, Vol. 37, 1892, PI. II, Fig. 3. 



