PLATE 57. 



HEART. 



Fig. 1. Sympterygi.\ acuta. M. C. Z. 632 (Page 370). Fig. 2. Disceus thayeri. M. C. Z. 

 297 (Page 426). Fig. 3. Pteroplatea altavela. M. C. Z. 336 (Page 415). Fig. 4. Aetobatus 

 NARiNARi. M.C.Z. 677 (Page 441). Fig.5. Rhinopteuajussieui. M. C. Z. 863 (Page 447). Fig.6. 

 MoBULA hypostoma. M. C. Z. 683 (Page 453). Fig. 7-10. Ceratodus forsteri. M.C.Z. 9827. 



The majority of the more differentiated of the famiUes of the Platosomia have a larger number of 

 rows of valves in the conus than others commonly accepted as much lower in rank; that is, with increase 

 in specialization decrease in the number of valves does not obtain as regularly as in the Antacea. This 

 is substantiated by contrast of Rhinobatus, Plate 56, fig. 6, with Aetobatus, fig. 4, Rhinoptera, fig. 5, 

 and Mobula, fig. 6, of Plate 57. 



Figures 7-10 re{)resent the heart of Ceratodus. 



For fig. 7 the pericardium was slit longitudinally and turned to the sides; for fig. 8 both conus and 

 ventricle were opened, showing the upper end of the so-called spiral valve in the passage from the ventri- 

 cle into the conus, the lower end of the passage with the end of the hinder one of a series of large thick- 

 walled valves of the aditus, and the chamber of the ventricle with the basal, posterior, fibrous pad. 

 The conus is thrown open in fig. 9 disclosing the transverse series of large valves immediately behind the 

 arteries, and behind that series another transverse series of smaller valves; this figure also shows the 

 somewhat spiral course of the opening from the ventricle, in dotted lines, with the end of a large valve at 

 its origin, and the fibrous pad at the side of which is a large opening controlled by muscles into the ventri- 

 cle. The inner two of the dotted lines roughly indicate the position of a longitudinal series of large 

 valves laid bare in fig. 10 which shows one of the transverse series of large valves in front cut open to 

 prove that this series originated from one of the hinder of several transverse series, the smaller valves of 

 the anterior series being included by the larger valves; this figure also shows a longitudinal series in the 

 aditus of large valves one of which was formed from each of the transverse series of small valves, at its 

 left in the figure. 



