390 PEOF. G. B. HOWES ON THE 



arise very nearly midway between the ccsliac and Hyrtl's " pos- 

 terior mesenteric." The dotted line a — /3 of the figure will testify 

 to the truth of the assertion that, with respect to the origins of 

 these arteries, a gradational series is forthcoming among easily 

 accessible forms ; and, on the knowledge of that which has gone 

 before, we should be justified in removing the so-called " inferior " 

 or " posterior " mesenteric artery of the Plagiostomes from that 

 category, and relegating it to that of the superior mesenteric 

 series. 



The inferior mesenteric arteries of those Ichthyopsida least 

 remote from the fishes (i. e. the TJrodeld) are usually four in 

 number, and I have attempted to show {ante, p. 387) that the 

 reduction in number of these vessels met with among the higher 

 forms (ex. IBana 1 to 3, Lacerta 2 to 3, higher Amniota 1 or 

 more) may, in all probability, have been due to modification of 

 such a series by concrescence. In view of the variation of the 

 coeliaco-superior-mesenteric arteries before referred to, one is 

 led to ply the same question, viz. : may not they be the modified 

 derivatives of a closely related series ? That they are liable to 

 concrescence at the present day cannot be doubted, since, in 

 Acanthias (fig. 7, v.), it occasionally happens that the mesenteric 

 and lieno-gastric trunks unite. Among the Urodela we meet 

 with a condition of the arteries of the coeliaco-superior-mesenteric 

 series \iiyv\aQh.{Salamandra,^g. 5) the several trunks lie {a.cm.sm.) 

 closely aggregated side by side. The most recent account of these 

 is that given by Wiedersheim (33. p. 175). Hyrtl has long ago 

 described them, as also a similar series in Proteus, Siren, and 

 Cryptolranchus (cf. ante, p. 385) ; Hofimann alludes to them 

 (13. pp. 493-494) as the arteriae " gastrica anterior," " gastrica 

 mesenterica," and " mesenterica primse et accessorise." Wieder- 

 sheim figures, in Salamandra, six main trunks ; I find seven to 

 be the usual number * (fig. 5, a.cm. sm.). Be there six or seven 

 present, the posterior one invariably supplies the head of the 

 large intestine; in its relationships to the inferior mesenteric 

 vessels it stands identical with the so-called hsemorrhoidal 

 branch of the Erog's superior mesenteric on the one hand, 

 and with the so-called inferior or posterior mesenteric of the 

 Plagiostomes on the other. The arteries of this very interesting 



* Hyrtl enumerates from 13 to 17 for the collective series in the Urodeles 

 referred to. 



