464 Bashford Dean Memorial Volwne 



This imaginary artery has been one of the causes operating to delay recognition of the 

 paired dorsal aortae. First described by Hyrtl (1872) in Catulus, its status and importance 

 were established by the author's prestige. Later Ayers (1899) reported the same vessel in 

 Chlamydoselachus, seemingly to place this artery on a firm basis. But many other workers 

 have since been unable to find any trace of it whatever in any species, either in embryo or 

 adult. Dohm attempted to explain Ayers's paper but only confused matters the more, and 



Text-figure 108. 



The dorsal aorta (anterior portion) and its branches, also the first efferent branchial artery 



and its branches, in Heptandms maculatus. 



ac., anterior cerebral; a.sp., arteria spinalis; hr.ef.l, first branchial efferent; d.a.I, paired dorsal aorta; d.a., dorsal 

 aorta; hy.ef., hyoidean efferent; i.e., internal carotid; m.c, median cerebral; ns., nasal artery; o.m., ophthalmica 

 magna; or., orbital artery; p.c, posterior cerebral; ps., pseudobranchial artery; r.a., ramus anastomoticus; rs., 



rostral artery; sg., segmental artery. 

 After DanieL 1934, Fig. 152. 



it remained for Allis (1911.2) to re-examine the same species and to expose so many other 

 glaring errors in the pre\dous work that Ayers's description has been entirely discredited. 



These paragraphs furnish the most striking case encountered in this investigation 

 illustrating the danger of (l) erecting specific types from the dissection of a single specimen; 

 (2) not making adequate allowance for a possible high degree of variability; and (3) attempting 

 to establish adult homologies without thorough embryological preparation. 



The bifurcation of the dorsal aorta anteriorly, as portrayed in Text-figures 107, 108 

 and 109, of Chlamydoselachus, Heptanchus and Squalus respectively, is a feature common 

 to all elasmobranchs, so far as knov.Ti. From an embryological point of view this is 



