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particularly interested, the carotid arteries, and so was led, first to 

 look up the literature of the subject, and then to begin the series of 

 short monographs of which this present one is the fourth. 



In the drawings sent me by Mr. Allen, the relations of the 

 cranial vessels to the skeleton are not shown, and as those relations 

 are important for a proper understanding of the vessels, I have had 

 serial sections prepared of several larvae of this fish, ranging from 

 130 mm. to 170 mm, in length; and the descriptions given in the 

 following pages, unless otherwise stated, are based entirely on a study 

 of these sections, and on certain dissections made in my laboratory 

 here of adult but uninjected specimens. These sections and dissections 

 were prepared by my assistant, Mr. John Henby, from material sent 

 nie by Mr. Allen; and later, when my work was necessarily inter- 

 rupted by long and repeated absences, Mr. Henry made camera 

 drawings of certain series of the sections, traced all important structures 

 through them, and later made reconstructions of all of the vessels 

 herein particularly described. 



Back of the branchial region, I have not attempted to study the 

 vessels, and the few references made to vessels of that region are 

 based wholly on the drawings sent me by Mr. Allen. Reference is 

 however frequently made to the cranial vessels as shown in Mr. Allen's 

 drawings, for the conditions found in larvae frequently differed from 

 those shown in the drawings of the adult. These difierences are, in 

 each case, noted because they may be due to individual variation. 

 It seems to me however much more probable that they are largely 

 or wholly due to insufficiency of the injection method; some of the 

 smaller vessels being quite probably and quite naturally overlooked, 

 if the injection mass failed to enter them. 



The pseudobranchial and carotid arteries, and the arteries in the 

 first two branchial arches, are shown diagrammatically represented in 

 the accompanying figure. 



The truncus arteriosus of the adult Polyodon is a long vessel 

 which separates, at its extreme anterior end, into two portions each 

 of which turns laterally downward and backward, and then upward, 

 and is the afferent artery of the first branchial arch of its side. 

 Slightly posterior to this point of separation, two vessels arise close 

 together on each side, one from the lateral, or dorso-lateral, and the 

 other from the dorsal surface of the truncus. The lateral vessel runs 

 laterally and backward, and then upward, and is the afferent artery 

 of the second branchial arch. The dorsal one runs upward, laterally 

 and backward and soon separates into two parts, one of which is the 



