498 PEOF. EAT LAKKESTEE ON THE REAETS OF 



tendency to individual variation in the number and character of the pocket valves of 

 the cone of Selachians and Ganoids, he was prepared to find that his two specimens 

 differed in the disposition of these parts. In the one there was found an upper row 

 of four large watch-pocket valves (close to the origins of the branchial vessels), each 

 thickened at its base by a vertical pad, such as Gegenbaur describes in AcantJdas ; and 

 below this series were observed two papillae placed side by side. These Dr. Giinther 

 took to be rudimentary indications of a second row of pocket valves. His second spe- 

 cimen proved this supposition to be correct ; for in it he found the upper row of four 

 pocket valves with their pads, and a second complete row of four small pocket valves in 

 place of the two papillae. 



The specimen examined by me has the valves in this part of the cone more strongly 

 developed than was the case even with Dr. Giinther's second specimen. The upper 

 series of four large pocket valves are present, and possess thickened pads which appear 

 as the bases of the pockets, not as separated tubercles such as are drawn in Dr. 

 Giinther's plate. The second series of four pocket valves is complete, and the valves 

 are relatively larger than in the second specimen figured by Dr. Giinther. It would 

 seem possible, from the amount of difference between my specimen and Dr. Giinther's 

 first specimen, that a considerable range of valve-development obtains in the Ceratodus 

 cone ; and it is even possible that the delicate valves of the lower vertical segment 

 of the cone do not exist, excepting in those specimens which are, so to speak, highly 

 valvular. 



The second point is one on which I venture to differ from Dr. Giinther. It appears 

 to me that the longitudinal fold in the conus, both of Ceratodus and Protopterus, is 

 not to be regarded as consisting of separate parts, viz. valve and muscle, but that the 

 whole fold is a muscular contractile ridge which acts as a valve in consequence of the 

 spiral disposition of the tube in which it exists. Dr. Giinther, however, describes that 

 part of the longitudinal valve which lies in the lower vertical limb of the conus as a 

 muscle, whilst only the twisted portion running into the transverse portion of the conus 

 is regarded by him as " valve." Functionally this may be true ; morphologically the 

 division is, I think, inadmissible. 



The third structural feature to which I will draw attention is the remarkable 

 " fibro-cartilaginous mass " first described by Hyrtl in Lepidosiren, which, originating 

 in the fioor of the ventricle, extends through the auriculo-ventricular passage, expands 

 into a large knob in the fioor of the auricle, and can then be traced further as a dense 

 fibrous ridge right across the fioor of the sinu-auricular cavity. 



This structure, and the fact of the complete continuity of " sinus " and " auricle," are 

 exhibited in the dissection fig. 4 (FC). A dotted line on the ventricular wall in fig. 3 

 indicates the direction of the deep cut which has now been made, the point of the 

 scissors being passed completely through the auriculo-ventricular passage. 



Dr. Giinther speaks of the sinus and auricle as distinct structures, the former open- 



