258 LECTURE X. 



Ganoid fishes,, and are arranged in longitudinal rather than in trans- 

 verse rows : the Polypterus shows three such rows of nine or ten 

 larger semilunar valves alternating with as many rows of smaller 

 valves. The Lepidosteus has five longitudinal rows of sub-equal 

 valves : those at the end of the bulb being always the largest and 

 most efficient. * In the Lepidosiren the place of valves is sup- 

 plied in its long and twisted bulbus arteriosus by two longitudinal 

 ridges (yfig- 71. c) j ; the interesting stages, which we have been tra- 

 cing through the highly organised Ganoids and Plagiostomes, in the 

 partition of the bulb into distinct arterial trunks for the systemic and 

 pulmonic circulation, being most advanced in this amphibious fish. 



The auricle in the Lepidosiren annectens is essentially single, but 

 has two ear-like appendages.;]: The venous sinus communicates with 

 it without any intervening valve ; the auricle receives the vein from 

 the air-bladder by a distinct aperture, close to the opening into the 

 ventricle ; regurgitation into the vein being prevented by a hard 

 valvular tubercle, which also projects into the ventricle.§ The ven- 

 tricle (ib. 6) is single, like the auricle ; its inner parietes are very 

 irregular: a 'trabecula' projects from the lower part of the cavity, 

 like a rudimental septum : a smaller transverse ' trabecula ' arches 

 over and acts as a valve to the single auriculo-ventricular opening, 

 but there are no proper membranous semilunar valves. 



The muscular pai-ietes of the ' bulbus arteriosus ' are distinct in all 

 fishes from those of the ventricle ; they may be overlapped by these, 

 but an aponeurotic septum intervenes between the origin of the 

 bulb and the overlapping ventricular fibres (see prep. No. 910.). 



BRANCHIAL VESSELS AND GILLS. 



The primary division of the branchial artery in the Myxinoids has 

 been already described. Each gill-sac receives, either from the trunk 

 or its bifurcations, its proper artery. The leading condition of the 

 gills in other fishes may be understood by supposing each compressed 



* XXV. pi. ii. t XXXIII. p. 343. pi. xxvi. fig. 2. c. | Ib. p. 345. 



§ A true second auricle consists essentially of a dilatation of this homologue of 

 the ' vena pulmonalis.' Hyrtl (cxxiv.) errs in stating that the fibro-cartilaginous 

 tubercle below the auriculo-ventricular aperture was " weder von Bisclioff noch 

 Owen angegeben." It is described in my Memoir (xxm. ), p. 345. : " It empties 

 Its contents into the ventricle by a distinct orifice, protected hy a cartilaginous valvular 

 tubercle ;" and the tubercle is figured, from the auricular side, in pi. xxvi. fig. 2., 

 where a bristle is placed above it, as in fig. 71. Hyrtl gives a figure of the same 

 'cartilaginous tubercle' as " dick en eiformigen harten faserknorpel " (p. 35. ), or 

 " fibro-cartilaginbse Stempfel" (p. 60.), from the ventricular side, tab. i. fig. 3. c. 

 It is true that this singular body escaped the notice of Dr. BischoflT, who, believing 

 that the Lepidosiren was a reptile, overlooked many things that were, and saw 

 some things that were not, in its organisation ; as, e. g. two distinct auricles, and a 

 nasal meatus communicating with the mouth. 



