G8 William F. Allen 



about the same as in the Teleosts.'^^ Like Aniiatus, the caudal sinuses 

 of Lepisosteus are situated ventrad and to the side of the last vertebrge. 

 They do not lie in a longitudinal plane, but are tilted a little obliquely 

 ventrad, following the general contour of the vertebral column in that 

 region. Usually they cross the vertebral bases of the eighth and ninth 

 or the ninth and tenth caudal hgemal spines (counting ventro-dorsad). 

 These reservoirs are somewhat elongated, ordinarily deeper anteriorly 

 than posteriorly, but in some instances, as shown in Figs. 1 and 4, they 

 have little the appearance of sinuses, but rather maintain the same 

 diameter throughout, which is slightly, if any, greater than the adjacent 

 subcutaneous trunks. The contents of these sinuses was not examined, 

 nor were they sectioned, save in a 90 mm. L. osseus and a few smaller 

 specimens, where they were found to be composed of a single layer of 

 endothelium, not unlike the longitudinal subcutaneous trunks or the 

 caudal vein, and contained, especially, at their posterior ends and their 

 junction with the caudal vein, a mucous or plasma-like substance, in 

 which there were a few red and white corpuscles. 



Considerable has already been said concerning the various openings 

 of the caudal sinus. There is always a communication between the two 

 sinuses (Figs. 1-7, o), wliich passes between the eighth and ninth or 

 the ninth and tenth caudal haemal spines (counting ventro-dorsad). 

 The aperture of this connection is through the mesal wall of the sinus, 

 at about its center near the floor, but in the specimen from which Figs. 

 1 and 2 were drawn it was nearer the roof. N"o valves were found 

 guarding this orifice, and in Fig. 17 (o) this communication is shown 

 in cross section. One or the other of the caudal sinuses, more often the 

 right, receives the posterior continuation of the ventral trunk or what 

 has been described as the posterior trunk (Figs. 1-5 and 7, C. T.) from 

 the rear. As has already been stated, each of the sinuses (x) (Figs. 

 1-6) joins its respective caudal sinus at the dorso-cephalic corner; while 

 the opening through the ventro-cephalic corner leads into the caudal 

 vein (see Figs. 1-7 and 15). The venous opening of the caudal sinus 

 that receives the caudal trunk is always much larger than the other, 

 as is shown in Fig. 7. Unlike the other orifices of the caudal sinus, the 



"In both Lepisosteus aud Amiatus we have a masked heterocercal tail, 

 which is more primitive than the tails of bony fish in that the caudal haemal 

 and iuterhre-mal spines, although fused to each other, have not fused together 

 in forming two hypural Imnes, as is the case in the Teleosts. 



