'944 DR. W. G. RIDEWOOD ON THE [NoV. 28, 



condition is more primitive than that found in Megalops (fig. 4), in 

 which the third efferent vessel unites with the fourth on each side 

 to form a short common trunk which carries the double charge 

 of blood to the aorta. If this line of argumentation be extended 

 to the cases in which the third and fourth vessels open into the 

 circulus cephalicus instead of into the aorta, we are led to the 

 conclusion that Blennius (fig. 35), having the Y-shaped system, is 

 more specialized than Gadus (fig. 34), where the third and fourth 

 vessels are disposed in the form of a V, and that this latter is more 

 specialized than SyngnatJms (fig. 33), where the two vessels find 

 separate outlets into the circulus. 



Whether much importance, however, can be attached to this 

 last feature is open to considerable question, for the V-shaped 

 system obtains in Salmo (fig. 7) and the Y-shaped one in Osmerus 

 (fig. 10), and a similar relation exists between the Chinese Carp 

 HiipopTithalmiclitlnis and our native Carp Cyprimis (fig. 13), while 

 among the Siluroids, a presumably natural assemblage of forms, 

 there are gradations from CallichtJiys (fig. 19) and Liocassis 

 (fig. 18) with the V-shaped system, through Clarias (fig. 20), and 

 Silurus (fig. 32), to Saccobranchus (fig. 31), with a typical Y-shaped 

 arrangement. 



The disposition of the efferent branchial vessels is independent 

 of the shape of the head, except in so far as the slope of the 

 vessels and the shape of the circulus is concerned. There is no 

 connection, that is to say, between the shape of the head and the 

 degree of suppression of the median aorta, or the entry of the 

 efferent branchial vessels into the circulus rather than into the 

 aorta. In long-headed forms like Ammodytes (fig. 3), Sphyrcena, 

 Fistularia (fig. 30), Anguilla (fig. 16), and Syngnathus (fig. 33) 

 the circulus cephalicus is elongated in an antero-posterior 

 direction ; while in Cottus (fig. 27), Lopliius (fig. 26), and others, 

 with a broad, flat head, the gills are widely separated, and the 

 circulus cephalicus is proportionately broad, the common trunks 

 formed by the fusion of the third and fourth efferent vessels 

 being also lengthened. 



Tlae differences in the arrangement of the efferent branchial 

 vessels relatively to the circulus cephalicus and the aorta are not 

 correlated with any differences in the position and extent of 

 development of the epipharyngeal dentition. At the commence- 

 ment of the inquiry, the suggestion occurred to my mind that the 

 development of a large and elaborate dental apparatus might, by 

 some process of natural selection, have resulted in the blood-vessels 

 taking up a position of safety, out of the line of direct pressure 

 between the epipharyngeal bones and the vertebral centra or the 

 base of the skull. A minute examination of the individual cases 

 shows, however, that the vessels do not experience any displacement 

 under the circumstances, but obtain sufficient protection by running 

 in grooves or arches in the epipharyngeal bones ; in fact, as often 

 as not, the epipharyngeal teeth lie immediately below certain of 

 the efferent branchial blood-vessels. As an instance of two forms 



