3l8 ALLEN 



(Fig. 12, D.Br.L. T.(s)O.) into which the third dorsal branchial 

 lymphatic trunk opens. 



In position the branchial lymphatic sinus corresponds some- 

 what to the so-called hyo-opercularis sinus and the anterior part 

 of the cephalic trunk of Polyodon. Its inclination to become 

 separated into two divisions recalls the two branchial sinuses of 

 Scorpanickthys (op. ctt., Pis. II and III; Figs. 4 and 5, 

 Br.L.S.), which were situated above the epibranchials of the 

 first and second, and the third and fourth arches, respectively, 

 but were some distance laterad of their position in Lepisosteus, 

 being located outside the branchial levator muscles, and the 

 space between the branchial levator muscles and the skull in 

 Scorp&nichtkys was occupied by the anterior continuation of 

 the abdominal sinus and the cranial lymphatic trunk together 

 with the jugular vein. In Ophiodon and Scorpanichthys no 

 branchial lymphatic trunks were observed, but in the former 

 nutrient veins were found emptying directly into the jugular. 



What is designated as the hyo-opercularis sinus in Lepisosteus 

 (Fig. n, Hyo.O.S.) is completely drawn on the right side only. 

 It is a reservoir of considerable size situated on the inner side 

 of the operculum, above the gills. So closely does it adhere to 

 the operculum that it is impossible to remove this bone without 

 rupturing its walls. This sinus is very much depressed and 

 conforms to the general curvature of the body. As in Scorpce- 

 nichthys it may be said to end dorso-cephalad in a papilla, which 

 in L. tristceckus expands into a distinct sinus immediately laterad 

 and cephalad of the first branchial levator muscle, to which the 

 name cephalic sinus has been given. 



This sinus (Figs. 8, n and 12, Ceph. S.) is joined from the 

 inside by the branchial sinus, and directly in front of this 

 point of union the cephalic sinus bends at right angles to pass 

 mesad and discharge itself in the jugular at a point about oppo- 

 site the cerebellum. With L. tristoechus the cephalic sinus has 

 much more the appearance of a reservoir than in L. osseus 

 (compare Fig. n with 12). In the latter it is nothing more 

 than a canal ; while even in L. tristwchus it falls far short of 

 its size in Scorpcemckthys, and is somewhat smaller than the 

 corresponding sinus of Polyodon. 



