68 MONOGRAPH OP DUEA DEN. 



The extreme termination of the tail of this specimen is not well shown, but 

 it is clearly displayed in that figured in Plate V. Here the dorso-caudal fin ends 

 posteriorly in a steep truncated margin, beyond which the persistent noto- 

 chord is continued for nearly two inches. The inferior lobe of the caudal fin, 

 on the other hand, is continued very much farther backwards. 



The narrow anal fin (Plate VI.) is attached 9|- inches behind the snout; it 

 measures 2^ in. in length, and is brush-like in form, being broader towards 

 the end than at the root, but tapering almost to a point at its free extremity. 



The anterior end of the mandible of one specimen displays a strong conical 

 tooth, and there is evidence that Phaneropleuron Andersoni attained eighteen 

 inches in length. 



The specimens figured in Plate V. and Plate VI. fig. 3, again, show far better 

 than that first described, those peculiarities in the internal organization of the 

 fish on which I have based its name. In the specimen figured in Plate V., 

 the persistent gelatinous notochord has left an impression three-eighths of an 

 inch thick anteriorly, which continues straight, and of about the same thick- 

 ness, for seven inches and a half; it then bends up at an obtuse angle, and, 

 with a slightly concave superior contour, tapers to the extremity of the tail. 



Above the notochord, from its anterior end to where it bends up, lie 

 numerous slender but well ossified neural arches and spines, which are so 

 bent as to be slightly convex posteriorly, slightly concave anteriorly. The 

 anterior ones are about an inch long, and are slightly expanded at both ends. 



A series of shorter, interspinous bones, at most three-eighths of an inch 

 long, occupy the interspace between these and the base of the dorsal fin. 

 These bones also are slightly expanded at each end, and disappear posteriorly 

 at the bend of the notochord. 



Below the notochord lie in confusion a number of curved, slender, well- 

 ossified ribs, one-twentieth of an inch in diameter, which cease to be traceable 

 posteriorly about five inches and a half from the anterior end of the specimen. 



A second fragment exhibits the opposite face of the middle region of an- 

 other fish of about the same size as that just described. The dorsal fin dimi- 

 nishes anteriorly, the interspinous bones becoming shorter, but the neural 

 spines, retaining their curvature, become longer and stronger, and the ribs in- 

 crease in length. 



A third fragment (Plate VI., fig. 3) consists of the anterior part of the body, 

 with the hinder region of the head, whose upper walls have disappeared, 

 leaving part of the opercular apparatus and of the pectoral arch. The ribs 

 diminish in length anteriorly, but they retain their characteristic forms and 

 their well-ossified structure. 



Professor Agassiz originally called this fish Glypticus^ but he did not de- 

 scribe it or define the genus, and as the name was already in use it must be 

 changed. The generic appellation which I have proposed expresses the most 

 striking character of the fish — the curious obtrusiveness, if I may so say, of 

 its ribs, arising from their complete ossification and the thinness of the scales.* 



The affinity of Phaneropleuron with the typical cselacanths is indicated not 

 only by its singular tail, ])ut by its persistent notochord, by its lobate pec- 



■* 'PavepSi, Tr\evp6v. 



